A subwavelength anti-reflective grating made from GaAs material is proposed, with optional wavelength and polarization mode and a working wavelength of 976 nm. On the basis of effective medium theory and membrane theory, the grating is initially designed. Parameters of duty cycle, ridge height and period of the grating are optimized and determined according to the rigorous coupled wave theory. Meanwhile, the influence of various parameters on the grating transmissivity is analyzed. The designed anti-reflective grating is of high transmissivity, 99.99% for TE mode and 99.86% for TM mode, respectively. In the range of 976 nm± 30 nm, the transmissivity is above 99% for both mode gratings, which meets the requirements of device application. The effect of non-rectangular shape of the grating caused by fabrication error on the transmissivity and dominant polarization mode is studied.