The anti-reflection sub-wavelength gratings are placed at the inner cavity of the micro-electro-mechanical system (MEMS) wavelength tunable vertical cavity surface emitting laser (VCSEL), top surface and bottom surface of the top distributed Bragg reflectors(DBR), respectively. The effects of the positions on the wavelength tuning range, the standing wave field and the resonant cavity wavelength are analyzed. The simulation results demonstrate that when the sub-wavelength grating is placed in the inner cavity of the MEMS wavelength tunable VCSEL, the largest wavelength tuning range can be achieved, which can reach 54 nm, while the largest wavelength tuning ranges for the other two structures are 40 nm and 33 nm, respectively. It is known by analysis of the standing wave field that the active region energy is 0.36% of the total energy when the sub-wavelength grating is set on the top surface of the top DBR, while it is 0.08% and 0.02% when the sub-wavelength gratings are set in the inner cavity and on the bottom surface of the top DBR. In addition, the resonant cavity wavelength with the corresponding to the transverse electric (TE) and transverse magnetic (TM) polarizations appears the separation of 8.3 nm and 10.1 nm when the sub-wavelength gratings are put in the inner cavity and on the bottom surface of the top DBR, while it has not separation for the top surface of the top DBR. Thus, the polarization stable principles of the three structures are different.