The effects of damp on frequency and phase were considered at the same time. A simple transfer formulation was denoted. The original system can be transferred as a new system which can be studied using MLP method directly. Using the MLP method, the bifurcation equations of primary resonance and 1/3 subharmonic resonance for strongly nonlinear Duffing system were obtained. For the first time, the MLP method has been used in studying the Duffing system with damp items and the application scope for MLP method is extended.