The zoom principle and primary aberration theory are used to study the design method of a mechanically compensated all-off-axis reflective three-gear zoom optical system with long focal length. The initial structure solving model for coaxial four-reflective mirrors zoom optical system is developed and based on the model, the adaptive variant probability genetic algorithm developed is used to address it and the partial initial structure parameters of the coaxial system is obtained; further, it is properly made off-center and tilt to remove the central obscuration issue of the coaxial optical system, and the aberration of the off-axial system is corrected using optical design software Zemax. Using the design method discussed above, an aspheric surface design is adopt in the optical surfaces of the system’s all-reflective mirrors, yielding a high imaging quality off-axis four-reflective mirrors zoom optical system with three-gear zoom focal lengths of 300 mm, 600 mm, and 900 mm. The results show that the method provides an effective measure for an off-axis all-reflective three-gear zoom optical system and that the system can be designed to meet practical needs.