The polarization test system for a vertical cavity surface emitting laser(VCSEL)is designed. With this test system,the threshold behaviors of VCSEL are studied. With the increase of driven current,two orthogonal polarization states are observed and their thresholds are different. According to the thresholds,the working region of the laser is divided into three parts. The analysis based on the non-polarization and polarization experimental results show that the threshold of the experimental curve from the non-polarization measurements is the threshold of P-polarized light; the threshold of the S-polarized light of the laser is observed obviously with polarization measurements; In the working region between the thresholds of P-polarized and S-polarized light,the degree of the linear polarization is higher than other working regions. The studies of this paper set the steady basis for further researches,test,and applications of a vertical cavity surface emitting laser.