A new method is developed to obtain the eigen frequency and steady state response of multi-DOF system with multi-frequency harmonic excitation. Based on the superposition principle， the steady state response is divided into several components with different excitation frequencies. Correspondingly， the spring force， external excitation force and inertia force of the system are also divided into a number of components similar to that of the response. The quasi-static equilibrium among the spring force， external excitation force and inertia force of mass based on D'Alembert's principle is imposed between the corresponding components of these forces. Then， the transfer matrix method is employed to attain the eigen frequency and steady state response of multi-DOF system without resolving the eigenvalue problem in advance. A 4-DOF system is used to demonstrate the proposed method， suggesting that application of the new method can easily obtain analytical solutions， which is verified by the modal superposition method and shows that the proposed method is also effective for obtaining the eigen frequency and mode shape of multi-DOF system without resolving the eigenvalue problem in advance.