Polarizer is an important optical device. Generally, the polarization direction of polarizer used in laboratory is unknown. In order to obtain the accurate polarization direction of the polarizer, a method for measuring the polarization direction of the polarizer based on Brewster's law is proposed. The polarizer with the measured polarization direction is placed in the optical path of the liquid crystal photoelectric test system, and the variation of the light transmission with the angle between the polarization direction of the polarizer and the front friction orientation direction of the liquid crystal cell is measured. The experimental results show that when the included angle between the polarization direction of the polarizer and the front friction orientation direction of the liquid crystal cell is 45°?, the light transmission reaches the maximum. At this angle, the change of the light transmission with the applied voltage and time is tested and analyzed theoretically. It is verified that the polarization direction of the polarizer can be accurately measured based on Brewster's law. Then a physical experimental optical path is built to measure the polarization direction of the unknown polarizer, which is verified by the known photoelectric test system, so that the polarizer can be better applied in the field where the polarization direction needs to be accurately measured.