In recent years, ultrafast fiber laser technology has become a core tool in modern photonics, promoting the rapid development of precision machining, optical communication, and biomedical imaging. Among them, mode-locked fiber lasers are capable of generating ultrashort pulses in the picosecond to femtosecond range, and their performance relies on SA to initiate and stabilize the pulse output. However, although traditional Saturable Absorber (SA) materials, such as Semiconductor Saturable Absorber Mirrors (SESAM) and carbon nanotubes, are widely used, they have inherent drawbacks: the modulation depth is usually less than 20%, and the saturated light intensity is not adjustable, which limits the implementation of higher-order harmonic mode locking. In order to achieve high-order harmonic mode locking, it is still necessary to rely on complex resonator design or external modulation techniques, which increases the complexity and cost of the system. In this context, Topological Insulators (TIs) are ideal candidates for high-performance optoelectronic devices due to their excellent nonlinear optical response and ultrafast carrier dynamics. Among them, bismuth telluride (Bi₂Te₃) has attracted extensive attention due to its narrow bandgap characteristics, high carrier mobility and significant optical nonlinear effects. The results show that Bi₂Te₃ exhibits a wide spectrum of saturable absorption in the near-infrared band, which can effectively control the formation and evolution of laser pulses, and becomes an ideal saturable absorber in ultrafast fiber lasers. Although TIs exhibit excellent nonlinear optical properties, their application in ultrafast photonics still faces significant challenges in materials science and device integration. First, the fixed bandgap of intrinsic Bi₂Te₃ limits its ability to manipulate key parameters such as modulation depth and saturation intensity, which are critical for optimizing laser performance under different pumping conditions. Secondly, the existing strategies to improve the performance of SA often sacrifice the stability and scalability of the material due to the complex preparation process and uneven film thickness. Therefore, intrinsic Bi₂Te₃ materials are difficult to meet the needs of mode-locked lasers with high repetition rate and high stability.At present, the optimization of fiber lasers mostly focuses on the design of gain medium and the control of resonator parameters. In terms of improving the performance of SA materials, the research is mainly achieved through nanostructure engineering. For example, two-dimensional materials (such as graphene, transition metal sulfide MoS₂) or quantum dot composites are prepared by mechanical exfoliation. Taking graphene as an example, its modulation depth can be increased to about 40%, but there are problems such as high unsaturation loss and poor thermal stability. Although MoS₂ has a high damage threshold, its nonlinear absorption coefficient is significantly lower than that of topological insulators. In addition, the mechanical stripping method is complex and has poor repeatability, which makes it difficult to meet the needs of large-scale device integration.In order to solve the above problems, this paper proposes to adjust the optical absorption characteristics of TIs through elemental doping. Compared with traditional doped elements, selenium (Se) and tellurium (Te) are both chalcogenides, and their smaller atomic radius and stronger electronegativity can effectively adjust the lattice structure and band characteristics, thereby optimizing the optical absorption performance of the material. In this paper, the regulatory mechanism of Se-doping on the structural changes and optical absorption characteristics of Bi₂Te_3-xSe_x nanosheets was systematically studied, and Bi₂Te_3-xSe_x nanosheets with different Se-doped concentrations (x=0, 0.3, 0.6, 0.9, 1.2, 1.5) were successfully prepared by solvothermal method, and the material ratio was confirmed by X-ray Photoelectron Spectroscopy (XPS), and the experimental components were in good agreement with the theoretical prediction. X-ray Diffraction (XRD) and Raman spectroscopy showed that Se doping led to lattice shrinkage and verified that the Te atom was replaced by Se. The absorption spectra showed that the absorption performance of the material was correlated with the change of wavelength, and the absorption rate showed a decreasing trend with the increase of wavelength. The effect of Se doping on the nonlinear optical absorption characteristics of the sample was tested and analyzed by the double-arm equilibrium detection method. The experimental results show that with the increase of Se doping concentration, the modulation depth of the material can be adjusted from 6.28% to 58.23%, the saturated light intensity ranges from 11.82 to 47.58 MW?cm-2, and the unsaturatedloss decreases to 30.76%. Furthermore, the optimized sample (x=1.2) was compounded with sodium Carboxymethyl Cellulose (CMC) to form an SA thin film, which was fixed between the fiber patch cord heads by flanges as mode-locked devices for fiber lasers. Under the condition of pumping power of 220 mW, a stable 12th-order harmonic mode-locked pulse output is realized, with a repetition rate of 230.46 MHz, a pulse width of 750 fs, and a signal-to-noise ratio of more than 45 dB. Compared with the traditional intrinsic Bi₂Te₃-based SA, this study realizes the active regulation of the mode-locking order through elemental doping, and does not need to rely on complex cavity design, which provides a new solution for ultrafast lasers with high repetition rate. In addition, this study not only provides a scalable synthesis path for tunable SA materials, but also provides a new idea for the functionalization of topological insulators through doping strategies. In the future, this strategy can be extended to other topological insulator systems, and doping-dependent pulse interaction dynamics can be explored to refine the theoretical model.