Abstract: The measurement of polarization rotary angle for faraday mirror based on polarization-beam-splitter was proposed. The orientation of the light from the faraday mirror was changed through a non-polarized beam splitter (NPBS). With a polarized beam splitter (PBS), the light reflected from NPBS was split into S light and P light. Their intensities were measured, and the theoretical expression of rotary angle was deduced after the impact of optical fluctuation is eliminated by signal processing. The influence of non-zero extinction ratio and polarized-angle error on output results was analyzed. Finally the intensities were repeatedly measured, and the average rotary angle was 89.68, the mean square deviation of all the experimental data was 0.01493.