Lithium-ion batteries are extensively utilized in portable electronic devices, electric vehicles, and large-scale energy storage due to their high energy density, long cycle life, and various other advantages. However, the limited natural abundance of lithium resources and their uneven geographical distribution imped the further development of the lithium-ion battery industry. Sodium-ion batteries have an application potential in large-scale energy storage due to their advantages such as abundant sodium resources, low cost, and compatibility with existing lithium-ion battery production lines. Replacing the conventional electrolyte with a solid electrolyte possessing flame retardant properties can effectively address the issues of thermal runaway and explosion associated with sodium-ion batteries. All solid-state sodium batteries (ASSB) offer benefits, including high energy density, enhanced safety, and low cost, aligning with the development goals of energy storage. As a critical component of ASSBs, the electrochemical properties of solid electrolytes play a pivotal role in determining their performance. Composite solid electrolytes (CSE), characterized by a good flexibility, high interfacial compatibility, and ease of processing, are considered as the most promising solid electrolytes for future large-scale commercial applications.This review summarizes recent research progress on organic-inorganic composite solid electrolytes (CSE) for ASSB and further analyzes the ion transport mechanisms within CSE. CSE are primarily composed of an organic polymer matrix, inorganic fillers, and sodium salts in specific ratios. The polar groups (i.e., −S−, C≡N, −O−, C=O) present in the polymer form group-ion complexes with the dissolved sodium salt. Within the amorphous regions, individual segments of the polymer chains exhibit relative freedom to rotate and bend, facilitating the movement of group-ion complexes within a limited spatial domain. As the chain segments reposition themselves appropriately, the group-ion complexes on these segments begin to segregate, allowing ions to interact with the functional groups of adjacent chain segments, thereby forming new group-ion complexes. This process is reiterated to facilitate ion transport. It is widely accepted that Na⁺ conduction predominantly occurs in the amorphous phase regions of the polymers above the glass transition temperature. In addition to these amorphous regions, certain crystalline domains exist within the polymers. In inorganic solid electrolytes, ionic hopping migration serves as the primary ion transport mechanism, which are largely affected by defects within the crystal lattice. The complexity of the interfacial region is further compounded in the presence of inorganic fillers that possess highly reactive surface defects, which readily interact with the polymer matrix. Two main explanations for ion transport in the interfacial region are proposed, i.e., 1) the interaction between the functional groups on the surfaces of the inorganic fillers and the polymer matrix, as well as the sodium salt, which weakens the interaction between the polymer matrix and Na⁺, resulting in a higher concentration of free Na⁺ on the filler surfaces and the formation of ion-transport channels, and 2) the space charge layer effect arises from the disparity in Na⁺ concentration between the inorganic filler and the polymer matrix, as well as the electrochemical potential difference. This results in the spontaneous formation of a Na⁺-rich space charge region, which serves as an efficient channel for Na⁺ transport. Solid electrolytes, including oxides and sulfides, are commonly utilized as active fillers, wherein the Na⁺ transport mechanisms encompass both vacancy and interstitial mechanisms.Based on their ionic conductivity, fillers are classified into inert fillers and active fillers. Inert fillers include Al₂O₃, ZnO, TiO₂, SiO₂, Y₂O₃, ZrO₂, MgO, and BaTiO₃, while active fillers encompass NASICON-type, calcite-type, and sulfide solid electrolytes. Fillers are further categorized by their shape and dimension into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures. Modifying the chemical properties of the filler surface enhances compatibility and interfacial bonding between the filler and the polymer matrix and allows for the modulation of ionic transport pathways within the electrolyte, significantly impacting overall electrochemical performance. Graft copolymers serve as interfacial modifiers, interacting with both the polymer and the filler to markedly improve interfacial bonding and enhance ionic conductivity. This strategy also mitigates the aggregation of fillers, which arises from the disparity in surface energy between the inorganic filler and the polymer matrix. Furthermore, the quantity of inorganic filler added plays a crucial role in ionic transport within CSE, making it essential to clarify the relationship between the amount of inorganic filler and ionic conductivity to effectively regulate ionic transport.Summary and prospectsComposite solid electrolytes (CSE), which consist of a polymer matrix combined with inorganic fillers, leverage the advantages of both solid polymer electrolytes (SPE) and inorganic solid electrolytes, offering promising prospects for practical applications. However, the development and implementation of CSE are still in their early stages, revealing a significant gap between their existing performance and application requirements, necessitating further enhancements. To address this, CSE with specific morphologies and microstructures must be designed to improve the connectivity of ion migration channels, facilitating efficient ion transport. An effective interfacial conductive network can be established via carefully controlling the ratio, size, dispersion, and other characteristics of the inorganic fillers, significantly enhancing ionic conductivity. The strategic application of simulation and characterization techniques to elucidate the ion transport mechanisms within CSE is crucial for advancing the performance of these materials. Lastly, to lower the costs associated with ASSB, the fabrication process of CSE should be streamlined to be simple, time-efficient, and material-saving, while also being compatible with the existing battery production lines to enhance the practicality.