Monolayer MoS2 is a promising semiconductor material for solar energy conversion application because of its excellent optoelectronic properties. In this study, monolayer n-type MoS2/p-type c-Si heterojunction solar cell was proposed and simulated using AMPS software. The different factors influenced the photovoltaic performance of the solar cell were studied. The simulation results show that the solar cell can achieve the highest conversion efficiency of 22.1% with the electron affinity of n-type MoS2 is 3.75 eV, the doping concentration of n-type MoS2 is 1018 cm-3, and the doping concentration of p-type c-Si is 1017 cm-3. Finally, the influence of interface states at the n-type MoS2/p-type c-Si heterointerface on the overall performance of solar cell is simulated. It is found that the interface state density over 1011 cm-2·eV-1 will seriously affect the photovoltaic performance of solar cell.