Abstract: A technique to measure the phase difference of wave plate in the process of wave-plate production is introduced. It is based on the laser feedback method and achieves the on contact block measurement, which means to measure the wave plate when it is bonded on the contact block. On the basis of achieving on contact block measurement, we studied the stabilization of laser cavity length, the influence of the radius of feedback mirror and the optical contact bonding layer. The system reaches the measurement uncertainty of 1 and the measurement precision of 0.5.