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For a caged spiral power spring, its moment of inertia varies with the time in energy storage and is closely related to the states of spring strip. Due to continuous changing of the shape in energy storage, calculation of variable moment of inertia for spiral power spring is a highly challenging but important issue. A general analytical model and calculation method of variable moment of inertia for spiral power spring are proposed in this article. First, the contraction process of spiral power spring in energy storage is clearly revealed and analytical model of spiral power spring on the basis of contraction process is presented. In terms of the contraction characteristic, a general calculation method based on divisions of stage and state for variable moment of inertia is put forward. Finally, a spiral power spring is designed, and a prototype of the spring is developed. The proposed model and general calculation method are applied into the prototype to calculate its variable moment of inertia, and they are evaluated by a developed experimental scheme.