In order to achieve a large image plane and large zoom ratio in microscopic imaging and solve the problem of the high integration of coaxial Kohler illumination, we propose a design method for a compact optical system with a large zoom ratio based on coaxial Kohler illumination. First, the imaging principle of the continuous zoom optical system of telescopes and microscopes is analyzed, and the design principle of the positive group compensation zoom microscope is analyzed theoretically. Then, the front fixed group is divided into a collimation group and a convergence group, and a beam splitter prism is designed between the two lens groups to achieve a compact coaxial Kohler illumination optical system by sharing lens groups. Finally, the continuous zoom microscope with a large image plane and the matched coaxial Kohler illumination optical system are designed. The design results show that the zoom ratio of the microscope optical system is 10×, the working distance is 60 mm, the highest resolution of the object side is 1.75 µm, and the coaxial illumination uniformity is 94.3%. The designed microscope has excellent imaging quality, minimal distortion, a smooth zoom curve, and a compact size, verifying the feasibility of the design method.