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A topological phase transition in high-temperature superconductor FeTe1−xSex, occurring at a critical range of Se concentration x, underlies their intrinsic topological superconductivity and emergence of Majorana states within vortices. However, how Se concentration and distribution determine the electronic states, particularly the presence or absence of Majorana states, in FeTe1−xSex remains unclear. In this study, we combine density functional theory calculations with pz–dxz/yz-based analysis and Wannier-based Hamiltonian analysis to systematically explore the electronic structures of diverse FeTe1−xSex compositions. Our investigation reveals a nonlinear variation of the spin–orbit coupling (SOC) gap between pz and dxz/yz bands in response to the Se concentration x, with the maximum gap occurring at x = 0.5. The pz–pz and dx2−y2–pz interactions are found to be critical for pd band inversion. Furthermore, the distribution of Se significantly modulates the SOC gap, thereby influencing the emergence of Majorana states within local vortices.