A highly efficient back reflector of amorphous silicon thinfilm solar cells, which is consisting of onedimensional photonic crystal and doublelayered twodimensional photonic crystal was designed by medium aSi∶H and SiO2. The structure parameters of the back reflector were optimized by calculating its reflectivity and transmissivity at the range of 300~1 100 nm by the finite difference time domain method. The optimal structure was obtained by the calculation and comparison of the short circuit current density in different structural parameters.The results show that the propagation path of photon in the cells can be extended and it can help to reduce the influence of the absorption layer′s thickness on the efficiency of the cells, and improve the absorption efficiency of the battery. So the highly efficient back reflector can greatly enhance the capacity of light harvesting and improve the short circuit current density to 31.96 mA/cm2, which is increased by 51.0% comparing with the solar cell with metallic back reflector of Ag/ZnO that is widely used now.