All-inorganic metal halides have shown significant applications in solid-state optoelectronics because of their flexible structures and impressive fluorescence emissions. In this study, a heterovalent cation substitution strategy was used to partially replace the divalent cadmium ions in CsCdCl3 with trivalent antimony ions to promote the production of self-trapped excitons, resulting in a bright broadband green photoluminescence of CsCdCl3∶Sb3+ with a central wavelength of 530 nm. Mechanism researches results show that the adjacent SbCl6 octahedra in CsCdCl3∶Sb3+ are isolated, forming a low-dimensional electronic configuration that promotes Sb3+ localization and achieves efficient photoluminescence with a quantum efficiency of up to 95.5%. Furthermore, although both CsCdCl3 and RbCdCl3 belong to ACdCl3 (A is an alkali metal family), they have distinctly different crystal structures. RbCdCl3 crystallizes in the orthorhombic crystal system with space group of Pnma; while CsCdCl3 crystallizes in the hexagonal phase crystal system with space group of P63/mmc. The structural symmetry of CsCdCl3 is higher than that of RbCdCl3, indicating that its crystal structure is less distorted away from the cubic phase than that of RbCdCl3, resulting in a smaller Stokes shift and corresponding blue shift of the emission spectrum in CsCdCl3∶Sb3+ than in RbCdCl3∶Sb3+. This work not only provides a method for designing new photoluminescence materials by heterovalent cation substitution but also paves an avenue for modulating the luminescent properties of metal halides through crystal structure symmetry.