The principle of linearly polarized light rotation based on Faraday modulation is analyzed. The results indicated that the output amplitude of fundamental frequency component is proportional to optical rotation θ.But the fundamental frequency component, which is less than zeroth-order and second-order frequency component, need be extracted by lock-in amplification technique. The ZF7 glass and TG28 crystal are used as the standard optical rotation and polarization modulation devices. The measurement system of linearly polarized light rotation is built by a lock-in amplifier. When the modulation frequency is 1.45 kHz and modulation magnitude is 0.035 rad, the experimental results show that the method can examine the angle of 6.3×10-8rad.