An optical system applied to the detection of 120～180 nm far ultraviolet waveband is developed for the study of ionosphere imaging spectrometer carried by satellite in China. The programme that uses off-axis parabolic as objective lens and Czerny-Turner system as imaging spectrum system structure is proposed after comparing different foreign programmes. The aberration theory is studied and a novel method of aberration correction is deduced to overcome the disadvantages of Czerny-Turner structure such as non-homogeneous aberration correction and low spatial resolution. Spectrograph system operating in 120～180 nm waveband with 4°field of view and 139.3 mm focal length is designed. The results demonstrate that the aberration of the system is substantially corrected and the modulation transfer function (MTF) of total field of view is more than 0.6 in all waveband, which satisfies the requirement. Compared with foreign loads, the structure is more convenient and predominant.