The interferometer is core of spatially modulated polarization interference imaging spectroscopy, the key component of the interferometer is Savart polariscope. The throughput of spatially modulated polarization interference imaging spectroscope is directly determined by transmissivity in Savart polariscope, thus affects forming the interferogram and target image. There is necessary to study the influence of transmission in Savart polariscope on throughput. After introducing the structure of interferometer, on the basis of electromagnetic field boundary conditions and the law of refraction-transmission and light intensity were obtained in Savart polariscope of the four interfaces: air /the left plate of Savart polariscope; the left plate of Savart polariscope/adhesive; adhesive/the right plate of Savart polariscope; the right plate of Savart polariscope/air, at the same time the light intensity is got after the two light beams interfere in through interferometer.The interference overall intensity is obtained by using light superposition principle,the mathematical expression of the transmissivity and throughput is solved. Through the relationship between the transmissivity and throughput, the influence of transmissivity in Savart polariscope on throughput can be intuitively known, this may provide a theoretical basis for the study of spatially modulated polarization interference imaging spectroscopy.