High efficiency, stability, and environmental sustainability are key objectives in the application of solar cells. Cu3SbS4 has received widespread attention due to its low manufacturing costs, environmental friendliness, and excellent optoelectronic properties. However, the small bandgap width, poor crystal quality, and improper band alignment have so far resulted in a power conversion efficiency (PCE) of only 0.46% for laboratory Au/Cu3SbS4/CdS/ZnO solar cells. In the present study, combining first-principles calculations and device simulations, three strategies to improve the performance of Cu3SbS4-based solar cells are proposed: (1) alloying with V, Nb, and Ta elements to increase the bandgap width of the absorber; (2) replacing buffer and window layers to optimize the band alignment of the device; (3) optimizing material synthesis and device fabrication processes to reduce series resistance and enhance shunt resistance. First-principles calculations demonstrate that Cu3Sb1–xAxS4 alloys (A = V, Nb, and Ta) are easy to prepare, and their bandgap widths can be widened while maintaining the original famatinite structure. Device simulations further reveal that the PCEs of the solar cells based on Cu3Sb1–xAxS4 alloys are significantly improved by optimizing the band alignment, reducing the series resistance, and enhancing shunt resistance. Finally, the device configurations Au/Cu3Sb0.75A0.25S4/SnS2/ZnO:Al are demonstrated to achieve PCEs of 19.55%, 19.67%, and 19.99% for A = V, Nb, and Ta, respectively. This study provides a theoretical foundation for optimizing Cu3SbS4-based solar cells and highlights the importance of combining theoretical calculations with device simulations to enhance the performance of solar cell devices.