Abstract: In order to accurately measure the ellipticity angle of composite wave plates, a method was proposed based on Mueller matrix. Firstly, a polarization model of the combined wave plate was established. Then, by measuring its Mueller matrix, three parameters, including retardation, fast axis azimuth and ellipticity angle were obtained simultaneously with non-linear fitting. Finally, the ellipticity angles of the combined zero-order wave plate with <i<λ</i</4 and <i<λ</i</2 were calculated and measured respectively by using the Mueller matrix ellipsometry system. The results show that the fitting error based on this method can achieve within 0.004, and the ellipticity angle and fast axis can achieve within 0.004. The measurement error of azimuth angle is 0.11° and that of phase delay is 0.22°. It is also found that the ellipticity angle and fast axis azimuth of the combined zero-order wave plate oscillate with wavelength, and the oscillation amplitudes of the combined zero-order wave plate are 1° and 0.4° respectively. This method is suitable for ellipticity angle measurement of arbitrary combined wave plates and has a wide application range.