Abstract: In allusion to the specific structural characters of Offner imaging spectrometer, the fundamental design principles of its fore telescope were analyzed. A novel method to design the wide FOV, long focal length and telecentric off-axis three-mirror anastigmatic (TMA) optical system was presented. For the projective demand, a fore telescope system for Offner imaging spectrometer was designed. Its focal length was 800 mm. Its FOV was 8.60.148, and the operation spectrum band was from 0.9 m to 5 m. The evaluation results of its imaging quality demonstrate that the MTF of each spectrum band is higher than 0.65 at the Nyquist frequency 16.7 lp/mm. The imaging quality has reached the diffraction-limitation, which satisfies the pre-designed requirement. Therefore, the design method is feasible. The designed telescope system is compact, easy to be fabricated and suitable for space borne Offner imaging spectrometer.