The relationship between the extinction ratio and the stress birefringence phase difference is identified based on a Jones matrix. The extinction ratio of a crystal is measured by the rotating sample method, its expression is also derived, and the errors are analyzed theoretically. A light source with power instability lower than 0.2% and a polarizer with an extinction ratio higher than 50 dB are used to build the test system. This test system is suitable for testing the samples with phase differences in the range of (π/2, π), and the test accuracy is less than -55 dB. The measured extinction ratio of a 1/2 wave plate is -41.66 dB, and the comprehensive error is less than 1%.