The polarization of vertical cavity surface emitting laser (VCSEL) can be controlled by electro optic birefringence.We calculated the birefringence resulted from external electric field,which was imposed on th top distributed Bragg reflectors (DBR) of VCSEL,by assuming that the two polarization modes were in the same place of the gain spectra in the absence of electric field beginning.By modifying spin flip model (SFM),the affection of the electric field strength on the polarization switching currents between the two polarization modes had been shown.