In order to get the simple computational formula for all movable zoom lens and reduce excessive dependence on designing experience of zoom lens, a new way to calculate optical-power distribution of zoom system is proposed.Taking the medium zoom position as initial parameter and the distances of groups as independent variables, the motion differential equations of all zoom positions is generated. With algorithm of partical swarm optimization under certain constraints, the best solutions are gotten, which are the positions of all groups in all zoom positions. This method reduces the difficulty of optical-power distribution in zoom lens system. What’s more, it provides a new way to get initial Gauss parameters of zoom system. A 100× Gauss structure with six components zoom system is designed, and the feasibility of this method is proved with it. The method can provide a practical and technical reference for all movable zoom lens system.