The core issue for first- order structure design of mechanical compensated zoom system is to determine the optical power and distance of components. In order to reduce excessive dependence on designing experience, a method is proposed for first- order structure design of continuous zoom lens system. Taking the optical power and distance of components as optimization variables, the motion equations are generated at different zoom positions. Regarding the image plane stability, total length of system and Petzval field curve as the evaluation function, the optimal solution is searched in multi- dimensional constraint space by particle swarm algorithm. In order to verify the feasibility of the proposed method, a first-order structure is optimized with the focal length ranging from 10 mm to 100 mm. This paper provides a new idea of solving the first-order structure of mechanical compensated zoom system.