Abstract: Y structure is the largest anticline structure in the middle of the Xihu Sag, while the exploration of Y structure has been in a stagnant state for many years. Through the analysis of abnormal high pressure, inclusions, authigenic illite, carbon isotope, physical properties and lithofacies of the failure wells Y1, Y4 and Y2 in Y structure, we sorted out the relevance of the various elements of hydrocarbon accumulation and re-understand the hydrocarbon migration pathway, hydrocarbon filling history, accumulation period and the fluid potential of the Y structure. The study shows that the descending plate of the large reverse fault on the west side of the Y structure is the most favorable for hydrocarbon accumulation.