Abstract: An exact modal synthesis method with the continuity of interface displacements and forces is proposed by using the mid/high-order modes of interface-fixed substructures. Substructure displacements are firstly expressed by the mid/high-order modes and exact residual constraint modes of the interface-fixed substructure, where the lower-order mode components are eliminated by the dynamic equation. Then the interfacial degrees of freedom are further eliminated based on the continuity of interface displacements and forces, and an exact reduced transformation is performed. The exact synthesis eigenvalue equation for structural mode vibration is obtained according to the Ritz procedure. Finally, the modal synthesis capacity and accuracy of the proposed method are verified by using numerical results in various synthesis cases.