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 group spacing as initial parameter and the actions of components as independent variables, the corresponding optical-power distribution and actions of components which satisfy the constraint of group space is get. This method reduces the difficulty of optical-power distribution and it is more important it provides us with another way to find the new varifocal moving mode. After getting the power distribution, we also deduce a simple way to obtain actions of each component. In order to prove the feasibility of this method, a four movable component zoom lens is demonstrated and the calculation result agrees well with the optimized result.