To overcome the shortage on dynamic range of the photoelectric detector, and the difficulties on accurate measurement of both the minimum and maximum intensity transmittances of the linear polarizer under the same condition, a method for extinction ratio measurement based on nonlinear fitting is proposed. The experimental system is setup. The measured polarizer is rotated only within a limited angular range to detect the transmitting intensities. Then, based on the principle of the model, the nonlinear fitting is applied on the obtained intensity data to calculate the extinction ratio. The experimental results show that the measured accuracy of 10^-5 level can be achieved. Also, good repeatability and effectiveness of the proposed method are verified.