A design method for a long focal length and co-aperture optical system with double field of view (FOV) and triple spectrum such as laser, visible light and medium-wave infrared band is described. An optical structure combining with off-axis three-mirror beam shrinkage and FOV switching is adopted in the front end of the system, which can realize the compression and FOV translation of large-aperture incident beam. A beam splitter group is adopted in the rear end of the system to divide the spectrum into three channels, which is excited into laser, visible light and medium-wave infrared imaging group respectively. And the combination imaging of each spectral band is realized. Through reasonable design, each spectral band has excellent imaging quality and can meet the requirements of the system.