A high-refractive-index contrast subwavelength grating （HCG） for 850 nm GaAs-based VCSELs was designed. The whole structure is based on GaAs material system，including a grating layer， a stress buffer layer designed to alleviate its stress problem and an AlO_x low refractive index sub-layer oxidized by AlGaAs or AlAs. The reflection characteristics of the HCG are simulated by Rsoft software， and the effects of different grating parameters on reflection spectrum are analyzed. In particular， the effects of stress buffer layer and low refractive index sub-layer on characteristics of gratings are investigated. The 850 nm TM mode HCG shows a very good reflection characteristic. It has a large reflection bandwidth of up to 91 nm with its reflectivity more than 99.9%， and the ratio with the center wavelength reaches 10.7%. Moreover， its reflectivity for TE mode is ensured to be lower than 90%， showing good polarization selectivity. This HCG can replace P-type distributed Bragg reflectors in VCSEL， providing high reflectivity， wide bandwidth， and good stability.