The light intensity transmittance of Glan-Taylor polarizing prism suffers from fluctuations in spatial incident angle. By using three-axis model and multibeam interference theory, we develop the expression of light transmittance of the prism in spatial incident angle. Based on Malus law, the light intensity transmittance of the analyzer in optical system is simulated, which shows that the fluctuations on intensity transmittance curve result from the multibeam interference in the parallel air gap, and the magnitude and position of the fluctuations lie on the parallelism of the air gap and the spatial position of the prism in the optical system. The intensity fluctuations can be completely suppressed with the angle between the rotation axis of the prism and the optical axis β=0°. For β≠0°, by giving the air gap a glued error angle, the transmittance fluctuations are well suppressed. The critical glued error angle for suppressing fluctuaitons decreases with the increase of the incident spot size.