Under varying intensities and polarization of the exciting light, the photoinduced birefringence of azo dye polymer film placed between a pair of orthogonal po larizers is studied by measuring the transmitted intensity. It is concluded that the saturation and stable values of the transmitted intensity are all proportio nal to the intensity of the exciting light. Both values are also related to the polarization direction of the exciting light, and when the angle θ between the polarization directions of the probe intensity and the exciting light is lag er than 10°, the saturation and stable values of the transmitted light are all proportional to sin22θ. In addition, the differences in birefringenc es of the sample were measured accurately by using a compensator and 1.11×10-3 (before excitated) and 3.57×10-3 (after excitated) are obtaine d.