The optimal design of Sagnac interferometer was studied to meet the lightweight design requirements of imaging spectrometer. The relationship between Sagnac and the magnification of system-s front telescope was discussed. The interferometer-s structure and optical paths were analyzed by unfolding it to flat glass. The experiential formula of the geometry dimensions of Sagnac that was changed with the field angle and the aperture of incident light was given. The optimal geometry dimension of Sagnac and the corresponding magnification of system-s front telescope could be calculated by the formula, which was called the optimal design. The optimal design result of a short wave spectrometer was calculated as an example using the formula.