The precise measurement of the birefringence in the liquid crystals has significant meaning for practical applications. The working principles of the liquid crystals were analyzed. The laser anisotropy external cavity feedback system was built based on the laser feedback effect. The anisotropy of the liquid crystals under different voltages was measured. The measurement results show that the accuracy of the laser anisotropy external cavity feedback system is within 0.3°; By imposing different voltage from 0 to 24 V, the birefringence changes from 2.74×10-1 to 2.39×10-3, corresponding to the large range phase retardation of 460° to 5°. With the voltage in the range of 0.7 V to 2 V, the relationship between the voltage and the birefringence is linear and its linearity is better than 95.5%. The liquid crystals can provide stable phase retardation, the short-term repeatability is better than 0.52° and the long-term repeatability is better than 4.5°.