In response to the phenomenon of petroleum downward migration exist in Chexi sag of Chezhen depression, herein, beginning from the analysis of the basic geological characteristics and combined with the oil-source correlation, and the distribution of effective source rock, and the position of the source rock and reservoir, the research showed that there does exist the phenomenon of petroleum downward migration below the burried depth of 4 700 m under the effective source rock of the third member of Shahejie formation(Es3) in this area. By means of oil-source correlation, it is confirmed that the oil in the pools of the fourth member of Shahejie formation(upper Es4)layer in the north of Caojiazhuan fault terrace in the sag is from the upper layer, the Es3 effective source rock in this area. On the basis of analysis of reservoir formation factors, it is clear that there are four main factors which control the reservoir formation by the downward migration. These main factors are the lithological relationship between sandstone in upper Es4 layer and the thick mudstone in the upper Es3 layer , the overpressure of the effective Es3 source rock, subtle passage system and the pressure difference between the reservoir and effective source rock. There is a positive correlation between the distance of petroleum downward migration and residual pressure in the effective source rock. The oil saturation was controlled by the physical characteristics of the reservoir under the source rock. The viewpoint above further improves the theoretical understanding on the reservoir formation in Jiyang depression and can be used as reference for exploration practice in Bohaiwan basin.