Abstract: In order to verify whether the thickness of air-gap would influence the prism's transmittance,the transmittance of Glan-Taylor prism and Glan-Faucault prism is theoretically studied.The transmittance of two-Glan-Taylor prisms and two-Glan-Faucault prisms is tested with a UV-3101 spectral photometer,and the results are found the transmittance of Glan-Taylor prism(about 85%) is obviously higher than that of Glan-Faucault prism(about 50%).For strictly collimating light,the transmittance of these two types of prism varies with the wavelength change,and this vibration is stronger for Glan-Faucault prism than for Glan-Taylor prism.However,the experimental result tested by spectral photometer doesn't have vibration,which shows that for non-strictly collimating light,the thickness of air-gap doesn't influence the prism's transmittance.