Back surface field (BSF) effect of polymorphous silicon with different band gaps on a-Si(n)/c-Si(p) heterojunction solar cell was simulated and analyzed by utilizing AFORS-HET software. It was predicted that the polymorphous silicon capable of producing the optimal BSF effect is the microcrystalline silicon with band gap of 16 eV，the doped concentration of 1018cm-3 and the thickness about 5nm. Such microcrystalline silicon BSF is easy to realize in practice. It makes the efficiency of solar cell much higher than that using the conventional Al BSF with the same doping concentration.