With the increasing development of light and small satellite technology, high-density optical payload has become a new hotspot research in space optical field. The field-bias and multi-light-channel coupled coaxial four-mirror optical system has the advantages of long focal length, large field of view and high degree of lightweight, which can satisfy the high-resolution, multi-function and low-cost applications of the optical payloads. Therefore, it has a widely application prospect in the field of light and small optical remote sensing payloads. Based on Gaussian optics and third-order aberration theory, the initial configuration of the coaxial four-mirror optical system was analyzed. An example of field-bias with visible spectrum and MWIR spectrum coupled optical system which was suitable for push-scanning was given. The visible channel has a focal length of 4 m, working spectrum of 0.45-0.85 μm, relative aperture of 1∶10, the MWIR channel has a focal length of 1.6 m, working spectrum of 3.7-4.8 μm, relative aperture of 1∶4, and the FOV of both channels are 1.4°×0.2°. The imaging quality and tolerance of the system were analyzed, from the results of the analysis, the imaging quality of each channel approaches the diffraction limit, the total length of the optical system is less than f’_visible/5.48, the system has higher compression ratio, and has easily implemented quality because of the relatively loose tolerance about processing and assembling.