In this paper, the influence of different front and back grating period ratio on the optical absorption property of a dual-grating structured thin film solar cell is analyzed using finite difference time domain method. Grating height, fill factor, and thickness of silicon layer are optimized, and the resonance modes that contribute to the absorption enhancement are studied. Results show that light leakage effect exists in the conformal grating structure with grating period ratio of 1:1, the structure with grating period ratio deviating from 1:1 suppresses the light leakage from the silicon active layer. The effective excitation and propagation of waveguide modes in the silicon active layer play an important role in the absorption enhancement of the structure with the front grating period smaller than the back grating period, while the excitation and propagation of the plasmonic modes in the active layer dominate the absorption enhancement of the structure with the front grating period larger than the back grating period. The light leakage effect also exists in the structure with grating period ratio of 1:2 and 1:3 when the silicon active layer is thick. As a result, the structure with the best absorption property deviates from the structures with these grating period ratios.