Abstract: The geometry characteristic of the fault system in the Xihu Sag shows a feature of "East-West zoning and South-North blocking". The structural style in the northern and southern parts of the West Slope Zone is of faulted terrace type; but on the Pinghu slope, the central part of the West Slope Zone, mainly is of single fault type. The structural style changes regularly from north to south in the Central Subsag-Inverted Structure Zone, indicating that the fault inversion intensity weakened gradually from north to south. In the Eastern Margin, the northern part is characterized by west dipping faulted terraces, but in the southern part they are east dipping, and the Huangyan fault zone, central part of the Eastern Margin, is a transitional zone. Specific phenomena have been observed in the transitional zone. For examples, E-W faults are well developed in the middle-south sag of the transitional zone, and NE, NNE faults developed on both sides of this transitional zone and pinch-out or displace laterally. In the middle-north sag, the transitional zone is characterized by the change in faults direction. The transitional zone between the north and south are basically follow the regional strike-slip basement faults in NW, NWW direction. The South-North difference of the Structural style in the Xihu Sag may be caused by the reactivation of the NW, NWW faults.