Abstract: Vertical migration along faults serves as the primary pathway for natural gas in the Xihu Sag, East China Sea Basin, where fault migration capacity controls the gas accumulation. The initial formation of most migration faults predates the hydrocarbon migration. During the basin inversion phase, these faults were inverted and reactivated as pre-existing faults, serving as migration pathway for natural gas. To understand the hydrocarbon accumulation conditions in reservoirs in the Y gas field of the Xihu Sag, the roles of fault geometric characteristics, fault-sand contact, and fault reactivation intensity during the migration period on vertical migration capacity were analyzed. The fault geometric characteristics and fault-sand contact area were determined by seismic interpretation; and the fault reactivation intensity was quantitatively characterized by fault slip velocity that derived from finite element numerical simulation. Through regression analysis of gas column height in reservoirs and the influencing factors on fault migration capacity, an index for quantitative evaluation of fault migration capacity was proposed, and applied in the Y and Q gas fields, showing good performance as an effective quantitative indicator to the fault migration capacity. This method provided a valuable reference and guidance for exploration activities in the regions with similar petroleum geological conditions.