To satisfy imaging spectrometers’s miniaturization, lightweight and large field requirements in space application, the current optical design of imaging spectrometer with Offner structure was analyzed, and an simple method to design imaging spectrometer with concave grating based on current ways was given. Using the method offered, the sub-field imaging spectrometer with 400 km altitude, 0.4~1.0 μm wavelength range, 5 F-number of 720 mm focal length and 4.3°total field was designed. Optical fiber was used to transfer the image in telescope’s focal plane to three slits arranged in the same plane so as to achieve sub-field. The CCD detector with 1 024×1 024 and 18 μm×18 μm was used to receive the image of the three slits after dispersing. Using ZEMAX software optimization and tolerance analysis, the system can satisfy 5 nm spectrum resolution and 5 m field resolution, and the MTF is over 0.62 with 28 lp·mm-1. The field of the system is almost 3 times that of similar instruments used in space probe.