A sub-wavelength transmission grating is designed and fabricated, which is utilized for the realization of diode laser array spectral beam combining. Based on the strictly coupled wave theory, the grating duty ratio, ridge height and period are designed, the effect of different structural parameters on grating diffraction efficiency is simulated, and thus the optimization of the grating diffraction efficiency is realized. The theoretical calculations show that, for a 940 nm laser, the -1 order diffraction efficiency of the optimized grating can reach 92%. The experimental results show that the output power is 46.2 W and the beam quality is 3.9 mm·mrad after the laser array spectral beam combining.