In this paper, we design a type of light trapping structure with an anodic aluminum oxide (AAO) nanograting on the surface and the backside of thin Si solar cells. Simulation of the influence of AAO's period, hole thickness, and duty cycle on thin Si solar cell's short current density are carried out by finite difference time domain method. Simulation results show that the optimum size for surface AAO is 0.5 for duty cycle, 75 nm for thickness, 440nm for period, and the optimum AAO size for rear AAO is 0.75 for duty cycle, 90nm for thickness, 380nm for period. The double AAO light trapping structure can increase the light absorption in the wavelength range from 280 to 1100nm，the absorption relative enhancement is 74.44%.