A real-time method for measuring the polarization state based on birefringence crystal wedge was proposed and it was applied to analyze the polarization state of transmitted light from twist nematic liquid crystal, the dynamic measurement of liquid crystal molecule mean director was achieved by this method. Polarization interference method based on crystal wedge was used to encode the polarization parameters of detected light into two groups of interference fringes, real-time measurement of polarization state was realized through stripe position location. According to the relationship between the director angle of liquid crystal director and transmitted polarization state, dynamic information of liquid crystal mean director can be derived. The measurement speed is 5 time per second in this experiment, and the accuracy of tilt and twist angle of liquid crystal average director is 0.2 degree. An effective and valuable approach for rapid measurement of liquid crystal molecule mean director is provided in this work.