In order to enhance light trapping in the crystalline silicon（c-Si）thin film solar cell, a cell structure that can significantly enhance light absorption is presented, which is composed of antireflection coatings, active layer, and back mirrors. Based on the basic principle of effective refractive index modulation and the rigorous coupled wave theory, optical properties of different layers are discussed through the numerical calculation and simulation, including the transmissivity of antireflection coatings, the reflectivity of back mirror, and the absorptance of the optimized c-Si thin film solar cell. Under the radiation of AM1.5G (solar spectrum including diffused reflection received at the earth surface with a 48° angle incidence) spectrum and for the c-Si thin film solar cell with 20 μm thick active layer, incident wave with TM polarization, and incident angle less than 75°, the integrated absorptance reaches 85.7%, 49%, and 14%, respectively when the wavelength ranging from 400 to 850 nm, 850 to 1000 nm, and 1000 to 1100 nm. It can effectively compensate for the shortage of near infrared absorptance in the crystalline silicon thin film solar cell.