Symmetric substructure dual gratings on the upper and bottom surface of the absorption layer are proposed to enhance the sunlight absorption for amorphous silicon thin-film solar cells. Using rigorous coupled wave method, it is found that in 600~750 nm wavelength range, the new gratings can effectively reduce the reflection from the upper surface of the incident light, the leakage transmittance from the bottom surface, and can enhance the absorption of amorphous silicon thin film solar cells with a thin absorption layer thickness of 400 nm. When the grating is adopted only at the upper surface, the optimized absorption can be increased by 71%; when it is adopted only at the bottom surface, the absorption can be increased by 24%; while for the case in which the gratings are adopted at both surfaces, the absorption can be enhanced by 81%. The study can provide new idea on designing high absorption and easily fabricated solar cells.