Light trapping is one of the key issues to improve the light absorption and increase the efficiency of solar cells. The absorption of Si thin film solar cells with different structures of back reflector, such as metal Al grating, inverse opal structure three dimension photonic crystal (3D PC) and their combination are systematically researched by utilizing finite difference time domain (FDTD) method. The results show that metal grating back reflector can mainly enhance the absorption of infrared light for solar cells. When the metal grating with the structural parameters of period is 700 nm, height is 100 nm, and duty cycle is 0.7, the average absorption rate of the solar cells in wavelength range of 750~1200 nm can be improved by 45.4% via replacing metal Al back reflector. For Si thin-film solar cells with inverse opal structure 3D PC the back reflector, the optimal structural parameters of air medium sphere are that the air medium sphere radium is 150 nm and the number of layers is 2. Compared to the metal Al back reflector,the average absorption rate of solar cells with 3D PC in wavelength range of 750~1200 nm can be improved a relative amount of 34.8% . Solar cells with combined structure back reflector has the best light absorption in wavelength range of 300~1200 nm. Compared to the metal Al back reflector, the average absorption rate can be improved a relative amount of 32%, which is better than metal grating or 3D PC. Due to the excellent reflection characteristics of metal grating and photonic crystals, the transmitted light is reflected back to absorbing layer, so that it can be absorbed two or more times. So the absorption efficiency of Si thin-film solar cells can be improved greatly.