A structure of the front semicircle and rear trapezoidal dual-interface grating monocrystalline silicon thin film solar cell was designed. The structure and the control group were simulated by the finite-difference time-domain method. It is shown that the dual-interface has better light-trapping performance than the single-interface grating structure by analysis of the short circuit current density and absorption spectrum. The absorption enhancement mechanism of the structure at long wavelength (750-1100 nm) was analyzed by using electromagnetic field distribution. In addition, for the front semicircle rear trapezoidal grating structure, the left and right slope of rear trapezoidal grating and the offset degree of front and rear gratings under the same period are further optimized. The results show that the irregular trapezoidal gratings have better light-trapping performance, and through the absorption efficiency cloud diagram, the best effect can be found when the offset degree is at 40 nm. The optimal short-circuit current density reaches 20.17 mA/cm², which is 58.1% higher than the planar structure by calculation and analysis. The research results have certain guiding significance for the grating structure design of thin film solar cells.