In this article，we investigate the effect of cathode work function，exciton generation rate and temperature as well as carriers and field distribution in the organic layer on the open-circuit voltage （Voc） of single layer organic solar cell with Schottky contact by using numerical method. It is demonstrated that the decrease of cathode work function （Wc） contribute to the increase of open-circuit voltage until Wc is close to the LOMO level of the organic material. The open-circuit voltage reaches a saturation value，when Wc is smaller then |EL|，and this open-circuit saturation voltage increases with the enhancement of exciton generation rate. For a given anode and cathode work function，the open-circuit voltage increases with exicton generation rate. When the exciton generation rate reaches a certain value，Voc goes to saturation instead of further increase，and the saturation value equals to the built-in voltage. Additionally，the saturated Voc decreases with the increase of temperature，and the decrease rate reduces with the increase of the exciton generation rate.