The effect of the parameters on the open-circuit voltage, VOC of a- Si:H/c-Si heterojunction solar cells was explored by an analytical model. The analytical results show that VOC increases linearly with the logarithm of illumination intensity under usual illumination. There are two critical values of the interface state density ( Dit ) for the open-circuit voltage (VOC),Ditcrit,1andDitcrit,2 (a few 1010cm-2?eV-1).VOC decreases remarkably when Dit is higher thanDitcrit,1. To achieve high VOC, the interface states should reduce down to a few 1010cm-2?eV-1. Due to the difference between the effective density of states in the conduction and valence band edges of c-Si, the open-circuit voltage of a-Si:H/c-Si heterojunction cells fabricated on n-type c-Si wafers is about 22 mV higher than that fabricated on p-type c-Si wafers at the same case. VOC decreases with decreasing the a-Si:H doping concentration at low doping level since the electric field over the c-Si depletion region is reduced at low doping level. Therefore, the a-Si:H layer should be doped higher than a critical value of 5×1018 cm-3 to achieve high VOC.