A relative angle method for measuring the phase retardation of wave plates is presented. Although it′s difficult to accurately determine the extinction position, this method has nothing to do with it. Compared with traditional methods, there is no need to find the extreme value positions to adjust the azimuth angle of the optical elements. And it can eliminate the influence of positioning error on the measurement results. This method not only reduces the difficulty of measurement, but also improves the accuracy of the measurement. Based on Mueller matrix, the theory as well as the measurement formula is derived. The phase retardation of 1/4 plate is measured when the suitable wavelength is 532 nm. The mean value and standard deviation are 90.072° and 0.42°,respectively. The usefulness of this method is verified by the result of the experiment.