Based on the large aperture diffraction telescope which is currently being studied, a scheme of optical system for diffraction telescope imaging in visible light range was proposed. The scheme solves the problem of narrow imaging spectrum existing in the current diffraction telescope and can obtain color images in the light range. The specific design methods are as follows: the diffraction elements are dividing into three channels along the radial direction, and the information of three color channels of R, G and B are imaged respectively,the imaging bandwidth of each channel is 40 nm;then by controlling the system parameters, the images of the three channels can coincide at the image plane and are acquired by the image sensor. A three-channel telescope optical system based on 25 m aperture diffraction primary mirror was designed and simulated. The simulation results agree with the design theory. The scheme can increase the spectral range of the system, and can get the same main lobe width as that of the single channel system.