Abstract: The phase trajectory analysis is introduced into the energy method for better angle damping estimate, in turn, the better roll motion prediction of a ship. The model test data are collected in the same way of the conventional energy method and processed with phase trajectory analysis for extracting amplitude information. A law of rolling amplitude decaying is determined through data fitting. The calculated rolling damping is verified through simulation with linear-plus-square damping moment model. The accuracy improvement gained by the method is demonstrated.