A highly efficient light-trapping structure consisting of a diffractive grating, a MgF2 film, a ZnS film and Ag reflector, is used for a-Si:H solar cell. Using the rigorous coupled wave theory, the weighted absorptance photon number (AM1.5) of a 1 m thick a-Si:H solar cell is calculated in a wavelength range from 400 to 1000 nm for the AM1.5 solar spectrum at 25 ℃. It is used to design the optimal parameters of the light-trapping structure. Results indicate that AM1.5 of the solar cell can reach 74.3%, if the period, the depth and the duty cycle of the diffractive grating, and the height of the MgF2 film and the ZnS film are 800 nm, 160 nm, 0.6125, 90 nm and 55 nm, respectively. If a ZnS/Ag film is fabricated on the rear surface of the solar cell, a larger AM1.5 (76.95%) can be obtained. It is demonstrated that the trapping structure is useful for elevating the efficiency of the solar cell.