Sb ₂(S,Se) ₃thin film solar cells have been developed rapidly in recent years due to their abundant raw materials, simple preparation method, stable performance, etc. In this study, based on the characteristic of tunable band gap of Sb ₂(S,Se) ₃light absorption layer, wx-AMPS software is used to simulate and design the Sb ₂(S,Se) ₃solar cell with narrowing band gap structure, and compared with the Sb ₂(S,Se) ₃solar cell with constant band gap (50% selenium content). The results show that the additional electric field formed by the narrowing band gap can promote the holes’ transport and inhibit the carrier’s recombination. Compared with the constant band gap structure, the narrowing band gap structure can increase the short-circuit current density of Sb ₂(S,Se) ₃solar cells from 19.34 to 22.94 mA·cm ^–2, the filling factor from 64.34% to 77.04%, and the photoelectric conversion efficiency from 12.03% to 14.42%. Then, the effect of electron mobility on the performance of Sb ₂(S,Se) ₃solar cells with narrowing band gap is studied. It is found that when the hole mobility is 0.1 cm ²·V ^–1·s ^–1, the advantage of narrowing band gap can gradually appear after the electron mobility is higher than 0.25 cm ²·V ^–1·s ^–1. The performance of Sb ₂(S,Se) ₃solar cell is enhanced with the electron mobility further increasing. However, when the electron mobility is higher than 5 cm ²·V ^–1·s ^–1, the device performance is saturated. Moreover, we demonstrate that the degradation caused by thick or high defect state of Sb ₂(S,Se) ₃solar cell can be effectively alleviated by applying the narrowing band gap due to the suppression of the carrier recombination. When the thickness is 1.5 μm and the defect density is 10 ¹⁶cm ^–3, the photoelectric conversion efficiency of Sb ₂(S,Se) ₃solar cell with narrowing band gap is 6.34% higher than that of the constant bandgap. Our results demonstrate that the band gap engineering of the light absorption layer is one of the effective technical routes to optimizing the performance of Sb ₂(S,Se) ₃solar cells. Since the photo-absorption material such as amorphous/microcrystalline silicon germanium, Copper indium gallium selenide and perovskite have the characteristic of tunable band gap. The design of the gradient band gap structure can also be applied to the optimization of the above alloy or compound solar cells.