Useing Czerny-Turner structure as the beam splitter a new optical design method of far ultraviolet imaging spectrometer used to get ionosphere data is presented. A source of aberration is analyzed. The method of aberration correction based on divergent illumination is proposed, which the imaging spectrometer is designed by. The wavelength range, field of view, focal length and F number, respectively, are from 120 to 180 nm, 0.1°×6.0°, 121.8 mm and 6.2. The designed system is evaluated by modulation transfer function (MTF) and imaging simulation. Its spectral resolution is better than 1.0 nm and its spatial resolution is better than 0.1°, which can meet with the requirements of ionosphere observation. Without any aspherical mirror, the optics is easy to manufacture.