Abstract: Xihu Sag is the largest petroleum-bearing sag in East China Sea shelf basin. Due to the large depth of the reservoir and the complex lithology and structure of the formation, the vertical well deviation problem is prominent. There is an urgent need to solve the problem of anti-deviation of vertical drilling. Based on the regional geology and drilling data, combined with the real drilling data and simulation calculation analysis, the vertical well deviation rules and reasons analysis were carried out. The deviation direction of vertical wells in this area is obviously affected by geological structure and generally conforms to the law of deviation along the updip direction of formation. The primary reason of serious well deviation in the lower sections is the large dip angle caused by strong tectonic action. Improper bottom-hole assembly (BHA) and drilling parameters are also important reasons. The results show that the upper Huagang Formation and above formations are not prone to well deviation, while the lower Huagang Formation and below formations are prone to well deviation. The deeper the borehole and smaller the borehole size, the more serious the deviation. Therefore, the vertical drilling system should be used in lower formations in this area, and pendulum drilling assembly should not be used or used carefully.