A new modal synthesis method is proposed in this paper for reducing the error caused by the higher order modal truncations. Through series expansion, the dynamic stiffness on substructure interface is expressed by the normal mode of the fixed-interface substructure. In addition, geometrical compatibility and force equilibrium on substructure interface are employed in the mode synthesis of the dynamic stiffness. Therefore a new methodology of substructure synthesis is obtained. The higher order force equilibrium on substructure interface is employed to achieve the model reduction. Compared to Craig-Bampton method, this method keeps the contribution of the higher order term of the truncated mode so that it can improve the accuracy of the mode synthesis. The derivation doesn’t introduce the assumption of the diagonal mass matrix which makes the method more universal. The numerical result is given to show the effectiveness of the method.