In this paper， a measurement method of stress birefringence based on photoelastic modulator is proposed and studied theoretically. Based on the measurement principle， the mathematical model is established. According to the mathematical model， the received signal is analyzed， and the calculation method of phase delay is proposed. In this method， DC component and first harmonic component are used to settle the phase delay， which can eliminate the influence of instrument constant on measurement results. Moreover， it is the most sensitive to the measurement of the phase delay near 90° and reliable for the sample with the delay between 45° and 135°. The numerical simulation analysis is carried out by using MATLAB. The wave plate with theoretical delay of 74.17° is measured， and the average measurement result is 74.28°. The experiment proves that the measurement scheme is accurate and reliable.