Far ultraviolet imaging spectrometer for upper atmosphere remote sensing is mainly used in observation on the far ultraviolet radiation and imaging for the mesoclimate phenomenon in the upper atmosphere. The foundation of correlated instruments is still frail in our country. A program in optical design is presented to improve the study of the imaging spectrometer for far ultraviolet observation in 130~180 nm. It is composed of the telescope with an off-axis parabolic mirror and the imaging spectrum system structure with tandem Wadsworth system. The tandem Wadsworth spectral imaging system adopts the collimator as an off-axis parabolic mirror. The light splitting device consists of a plane grating and a concave grating. The structure can realize quadrate dispersion. And the concave grating is used for focusing and imaging. Based on the aberration theory, the optical path function and aberration coefficients of the system are analyzed, and perfect imaging conditions of the improved structure are obtained. A design example using the perfect conditions is designed to meet the requests of the application in space observation in low orbit. The results demonstrate that the aberrations of the system are substantially corrected, and the modulation transfer functions in total fields of view and all waveband are more than 0.6. The improved structure is more convenient. The space resolution and the spectral resolution are both high.