Abstract: The exploration break through from Huagang Formation in the north segment of central anticline belt of Xihu Sag initiates geologists to re-think about the accumulation model in the deep Oligocene Huagang Formation. The analysis of gas source is one of the important aspects in the accumulation. The authors made a comparison among gas samples from the north segment of central anticline belt, Santan subsag and the north segment of Baochu slope, respectively, discussed the types of kerogen from the gas components and light hydrocarbon fingerprints, and demonstrated the gas maturity by a variety of indicators. It is concluded that the gases from Huagang Formation in the north segment of central anticline belt came from the same formation of Santan subsag which is located to the west of the central anticline, their maturity are inferior to those of gases from Huagang Formation in the south segment of Santan subsag and from Pinghu Formation in the north segment of Baochu slope. The false appearance, abnormal low content of heave hydrocarbon caused by fractionation and evaporation, can be easily mistaken for highly matured gas. The new idea about the gas origin put forwards a new method for the study on the accumulation model in Huagang Formation in the study area.