Exploring novel light-harvesting materials with excellent optoelectronic properties is crucial for photovoltaic technology. In this work, we investigate the optoelectronic properties of antimony selenides Na3SbSe4 using first-principles calculations and evaluate their photovoltaic potential by device simulations. The hybrid functionals predict a direct band gap of approximately 1.7 eV and effective masses of 0.549 m0 for electron and 0.591 m0 for hole. The light absorption coefficient is estimated to reach 105 cm−1 in the visible light range. Based on the spectroscopic limited maximum efficiency method, the power conversion efficiency is predicted to approach 19.58% with a thickness of 0.5 µm for light-harvesting material, revealing the excellent photovoltaic properties of Na3SbSe4. Device simulations further confirm that the solar cell with a device configuration of ZnO/Na3SbSe4/PEDOT:PSS can achieve an efficiency of 16.45%. Moreover, increasing the thickness of the light-absorbing layer and controlling the defect concentration can improve efficiency. These results can be significant theoretical guidance for the development of novel optoelectronic materials.