The mechanism of beam shearing splitting and principle of the interfering imaging of the self-developed onboard Polarization Interference Imaging Spectrometer (PIIS) were expounded in this paper. The throughput of PIIS is analyzed. The relation of the throughput with the angle of the polarized orientation of the polarizer to the ideal direction is derived. This Polarization Interference Imaging Spectrometer has the merit of simple structure, no moving parts, and high throughput. Because of its unique advantage of static, compact in size, wide field of view, it is applicable for the aviation, spaceflight, remote sensing, fieldwork or weak signal detection.