The imaging system with long focal length, multi-spectrum and light weight has the advantages of high resolution and small volume, which is an important research direction in imaging system design. According to the large shelter in cooled infrared coaxial three-mirror, an imaging system with the improved coaxial three-mirror is introduced. A piece of mirror is added behind the third mirror, the light from the system is reflected by the mirror. Off-axis aberration is corrected by second surface coefficients of three pieces of reflective mirror. The detailed design ideas and relative problems are given. A design example with 400 mm focal length, 4 F/# and 1.72° full field of view is given. The result shows that the imaging quality of the system approaches to diffraction limit.