Abstract: Force method equations adapt for an arch string structure with elastic support boundary are obtained by Castigliano theorem. Computing expressions for the internal force under a symmetrical distributed load and the pre- stressing force of the arch string structure are achieved through a symbol solution. These expressions reflect the relationship between the mechanical behavior of the arch string structure and structural parameters, such as vector high, cable-vertical distance, member stiffness and support stiffness. The expressions indicate that various structural parameters are not isolated, but they influence the structural mechanical properties by the ratio of each other. A practical arch string structure is analyzed by the theoretical method and finite element method, the accuracy of the theoretical method proposed is proved.