In traditional method of testing the extinction rate of polarizing prism, as the incidence is not vertical and the position of extinction can′t be located accurately, extinction ratio can be measured accurately neither.To solve this problem, taking Glan-Taylor prism for example, magneto-optic modulation was adopted to define the extinction place accurately, rotating the prism several times to decrease the nonlinear perturbation caused by oblique incidence. Simulation and experiment show that, the minimum and maximum light intensity can be gained, the extinction rate was also obtained and fulfilled the precision requirement.