As a type of special light source, microelectronic mechanical system (MEMS) vertical cavity surface emitting laser (VCSELs) has the advantages such as low power consumption, high modulation speed, broad wavelength tuning range and easy coupling, which has been widely applied in the field of laser communication. To achieve better performance of the lasers, such as enlarging wavelength tuning range and increasing polarization contrast, optimizing the structure parameters of inter cavity subwavelength grating is indispensable to improve the distribution of optical field and polarization output mode. In this paper, based on effective medium theory (EMT), the optimal values of period, duty cycle and ridge height of subwavelength grating are designed combining with the theory of film (the center wavelength of tunable range is 850 nm and the grating material is GaAs). The impact of grating duty cycle, ridge height on the transmission efficiency for TE and TM are analyzed. Furthermore, the tuning ranges of MEMS VCSELs with no-etched grating, unoptimized grating and optimized grating are compared. The results indicate that, for the specific wavelength tuning range and grating material, optimization of the grating parameters can promote the increase of the grating transmission efficiency for TE or TM mode, enhance the coupling of optical field between the semiconductor cavity and air gap, thus enlarge the wavelength tuning range of the laser.