A new parameter identification method for nonlinear multi-degree-of-freedom systems was presented. The two-degree-of-freedom systems with quadratic or cubic nonlinearities were studied. Based on the property that there exists energy integral in conservative systems, the Hamiltonian was derived and selected as the mathematic model of parameter identification. Hamiltonian function was fitted with the test data, which were the value sets of phase coordinates measured in free oscillation of the systems, and the parameters were identified with the least square method. No matter the nonlinearity of the system is strong or weak, the presented technique is valid as long as the system is conservative.