In order to meet the needs of broad band, high diffraction efficiency, and polarization independence, a double-layer trapezoidal polarization-independent beam grating is proposed in this paper. Firstly, based on the strict coupled wave theory, a design model of polarization-independent combined beam grating based on a particle swarm optimization algorithm is established, and the efficiency characteristics are optimized by randomly generating characteristic wavelengths. Then, the effects of slot depth, width ratio, side angle, and other structural parameters on the diffraction efficiency and bandwidth of single-layer and double-layer trapezoidal grating are analyzed in detail. Finally, the electric field enhancement characteristics of the two structures are analyzed and discussed. The results show that the polarization independent beam combining grating with double-layer trapezoidal groove achieves a theoretical diffraction efficiency of more than 99% in the bandwidth range of 51 nm (1038 nm-1089 nm), and has a larger process tolerance than the traditional single-layer trapezoidal structure, which meets the bandwidth of 30 nm and the high diffraction efficiency of 98% in the tolerance range, and has lower near-field grating enhancement. It has a stronger resistance to laser damage. The proposed double-layer trapezoidal grating with wide band and high diffraction efficiency can improve the output power of laser systems, and has significant application value in the field of laser beam combination.