The optical parameters of anisotropic crystal, with optical axis parallel to surface is measured by employing a single-wavelength reflective ellipsometer and method of rotating the sample many times. The principle of measuring the optical parameters of anisotropic crystal by single-wavelength ellipsometer is analyzed, the simulated annealing simplex joint inversion algorithm is improved, the influence of the light incident angle, sample optical axis direction, rotation angle, and positioning error on the measurement results are numerically simulated and analyzed. Experimental results show that the measurement accuracy of the main refractive index, absorption coefficient, and optical axis azimuth angle of the crystal are 0.0001-0.0003, 0.000001-0.000400, 0.02°-0.40°, respectively, which indicates the method has high self-consistency and accuracy.