Abstract: Laser feedback is a new method to measure the phase retardation of wave plate. The wave plate is located between the laser and the feedback mirror, which can make the laser feedback waveform produce 90o rotation of polarization, i.e. polarization flipping. The duty cycle of the two polarizations is related to the phase retardation. The computer will calculate the phase retardation automatically from the cycle duty. Through scanning the feedback mirror forwards and backwards and getting the duty cycle separately, we can eliminate the error caused by the fluctuation of the two polarization states′loss, and improve the long term stability. A repeatability of 0.5 degree is ensured. This device has advantages such as simple structure, high precision of on-line measurement, and can meet the need of industrial production.