A kind of amorphous silicon(a-Si) thin film solar cell structure, which has an antireflection (AR) coating and a back reflector, is presented. The AR coating consists of four dielectric materials with refractive index from low to high. The back reflector consists of a triangle dielectric diffraction grating and a one-dimensional photonic crystal structure. The parameters of the dielectric layers and the grating are optimized by rigorous coupled wave analysis and plane wave theory method. The reflection efficiency of the AR coating and back reflector with incident angle range of 0°～60° are calculated numerically. The results show that the AR coating has high transmission within the wavelength range of 300～750 nm and the back reflector has high reflection within the wavelength range of 600～750 nm. For the a-Si thin film solar cell with 700-nm-thick active layer, with incident wave of TM polarization and incident angle which is less than 75°, the solar cell has an average absorptance of 95% after optimizing in wavelength range of 300～750 nm.