The transmittance of Glan- Taylor prism (GTP) is a vital index to evaluate the component′s performance. Glan- Taylor prism is the key component of the polarization interference imaging spectrometer (PIIS). We adopt the Raying tracing method to deduce the theoretical transmittance expression of the GTP. By performing the computer simulation, some important relationship curves are obtained with the variations of the transmittance on optical axis and wavelength. The allowed range of the departure angles for the optical axis in the principle section are calculated. The results provide a theoretical guidance for the study and engineering of the polarization interference imaging spectrometer.