Abstract: Taking account of the theories of petroleum accumulation system(PAS) and petroleum accumulation dynamics, based on the characteristics of the PAS, the characteristics of the fluid dynamic field in C sag within the Weixinan Depression have been analyzed.Geological, geophysical and geochemical data were used,and basin modeling technique and petroleum geological colligation research methods adopted in the research. As the results,the petroleum accumulation processes were studied dynamically, and the hydrocarbon charging history was renewed. It suggests that the C Sag has higher geothermal gradient in the Second Member of the Liushagang Formation, with a fluid potential inherited from history. The sedimentation and tectonic subsidence show an episodic manner. The source rock of the Second Member of the Liushagang Formation had entered the period of mid-high maturity and had higher rate of hydrocarbon generation-expelling. The main episode of hydrocarbon generation-expelling happened after Miocene.It is believed that C Sag had passed several times of hydrocarbon charging. The Northeast part was charged earlier and the Southwest part later.