With the rapid development of perovskite solar cells (PSCs), environmentally friendly germanium-based PSCs have begun to appear in people's vision. We have designed a perovskite solar cell structure consisting of FTO/Cd0.5Zn0.5S/CsGeI3/MoO3/Au. Our study focused on analyzing the impact of various factors such as the absorption layer, electron transport layer (ETL), hole transport layer (HTL), interfacial defect layer (IDL), and temperature, on device performance. We optimized various parameters of the device, and as a result, the final PCE reached an impressive 35.91%. In addition, we conducted an analysis of the perovskite absorber layer using density functional theory (DFT) and investigated the function of Ge atoms in the perovskite absorber layer. The results show that, for HTL, altering the doping concentration of the ETL has a more significant impact on PSCs. When the operating temperature exceeds 300K, only the short-circuit current (Jsc) increases. When the defect density exceeds 1013cm−3, the rate of carrier recombination begins to increase significantly, along with the interface defect layer. We also analyzed the energy band, density of states, charge density difference, charge population, and elastic constants of the absorption layer using density functional theory (DFT). The results indicate that Ge atoms play a crucial role in photon absorption and photocurrent generation, which is a significant factor contributing to the high power conversion efficiency of germanium-based PSCs. Due to the transfer of electric charge from the Ge atom to the periphery of the I atom and the hybridization between the Ge-4p and I-5p orbitals, the Ge–I bond is formed and stabilized. Finally, the calculation of elastic constants shows that CsGeI3 is a stable mechanical structure.