With regard to the remote sensing, especially the subtle discriminating of the target, the imaging spectrometer with high delicacy, high spectrum resolution and super throughput are required. Based on the coaxial three-reflective optical system, an off-axial three-reflective optical system with no obstructing between three mirrors was designed by means of field of view off-axis form. The aperture stop of the system was on the secondary mirror, and with no middle image the requirement of high resolution, wide field of view, long focal length was satisfied. The capital parameters of the optical system are f′=1　600mm,2ω=18°×0.148°, F/number=F/10，and all surfaces are conicoid. The results indicate that the quality of imaging has arrived at the diffraction-limitation, and the optical system designed by this method has good application prospect on space sensing.