Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cell. In this paper, a novel combined light trapping structure consisting of back one-dimensional (1D) Ag nano-grating and front conformal antireflective coating is proposed for amorphous silicon (α-Si) thin film solar cell. By a numerical simulation based on the finite element method, the effect of the combination on the light absorption of α-Si solar cell is investigated, and the Ag nano-grating parameters are optimized. The results show that the combined light trapping structure can enhance broadband absorption in thin-film solar cell. For the α-Si solar cell with the combined structure at P=600 nm, H=90 nm, and W=180 nm, the integrated absorption is enhanced by 103% under AM1.5 illumination at normal incidence in a wavelength range of 300–800 nm, and the photon absorption rate is increased by 300% in a long-wavelength range of 650–750 nm compared with the reference cell. We discuss the physical mechanism of absorption enhancement in different wavelength ranges from the electrical field amplitude distributions in the solar cells. In addition, the solar cell with the combined structure is much less sensitive to the angle of incident light.