Abstract: Based on new results of regional study, play-evolving process of Xihu Sag was interpreted by means of structure histroy, oil/gas expulsion history, trace test of play-forming period and litho-facies data, and the occurrence time of a series of play-forming events was rediscovered such as trap (lithologic and structural traps) forming time, oil/gas expulsive periods, play-forming periods, oil/gas passage forming time and destructive effects, and the play developing process of Xihu Sag was analyzed. Finally, new point of view of play-forming was put forward in Xihu Sag.