Double perovskite solar cells have attracted much attention due to their low cost, high performance, environmental friendliness, and strong stability. In this study, the effect of thickness of perovskite layer, band offset, metal electrode work function, the thickness and doping concentration of the transport layer on the efficiency of Cs₂AgBi_0.75Sb_0.25Br₆ solar cells are analyzed by using Silvaco TCAD to improve device performance. This preliminary study of device based on Spiro-OMeTAD as hole transport layer (HTL) and ZnO as electron transport layer (ETL) shows that the photovoltaic conversion efficiency (PCE) is 12.66%. The results show that the efficiency gradually saturates when the thickness of the perovskite layer is greater than 500 nm. The optimal conduction band offset (CBO) ranges from 0 eV to +0.5 eV and the optimal valence band offset (VBO) from –0.1 eV to +0.2 eV. After changing the device's ETL into ZnOS and HTLs into MoO₃, Cu₂O and CuSCN, respectively, and optimizing their thickness values and doping concentrations, the final theoretical photovoltaic conversion efficiency of the double perovskite solar cell with an HTL of Cu₂O can reach 22.85%, which is increased by 25.6% compared with the currently reported theoretical efficiency value. Moreover, the optimal efficiency is achieved when the metal electrode work function is less than –4.9 eV. This work will help find suitable materials for the transport layer and provide guidance for developing the high-performance and lead-free perovskite solar cells.