To improve the computational efficiency of random excitation of large-scale structure and solve the dynamic problem of composite structures， a novel method for solving dynamic substructure is proposed. High-mass elements are added to the mass matrix of each substructure interface， and virtual excitation loads with different amplitudes are applied to these high-mass elements twice to ensure coordinated acceleration responses at the adjacent substructure interfaces. According to the internal force balance conditions and linear relations on the interfaces of different substructures， the interface force balance equations are established， the virtual excitation load amplitudes that can guarantee the coordination and balance of the acceleration responses and internal forces between each substructure are solved. The real response of each substructure is calculated by the obtained virtual excitation load.Compared with the traditional modal synthesis method， the proposed method avoids the conversions between modal coordinates and physical coordinates， and can consider the high frequency characteristics of the substructures. Compared with the mechanical admittance method， the proposed method reduces the scale of the synthesis equation and is more convenient to solve， and is also suitable for the case of simultaneous action of external loads and earthquake acceleration. This method is verified by different types of simulation examples.