In order to fix quarter-wave plate accurately, a new method is presented to measure the optical-axis azimuth of quarter-wave plate. The errors influencing test results are analyzed by Müller matrix theory. The main theories were used including Müeller matrix theory, Fourier analysis and least squares method, and the optimized Fourier coefficients of the signal are obtained by the least squares method. By the principle of extinction-ratio measuring system, the analyzing prism is fixed accurately. A high-accuracy step motor is used to control the rotation angle by the feedback loop system. The theoretical analysis of error proves that measurement error is related to the azimuth angle itself, and the experimental results have proved that the normal uncertainty in the measurement was 0.03.