As to improve the spectral resolution of the Sagnac interference imaging spectrometer, the mathematical model of the system is analyzed. The effect of the offset of the Sagnac prism on the spectral resolution of the system is discussed, and the concrete numerical value of the prism offset is calculated. The simulation experiment is carried out by importing the simulation spectral curve and assuming that it was error free in the instrument installation. The result shows that the spectral resolution can be 2 nm by adapting the calculated offset value, which has the guiding significance for the structure design of the system.