Abstract: A great breakthrough in oil and gas exploration has been made recently from the Huagang Formation in the central anticline belt of the Xihu Sag. Several medium to large oil and gas fields have been discovered. However, the gas column height and fullness ratio of trap differs from each other which suggests that every reservoir has its own key controlling factors. Based on seismic and drilling data, this paper selects the H3 reservoir as a case to study the accumulation conditions of source rocks, traps and combination of reservoir-cap systems. Also this paper sums up the key controlling factor of each reservoir by dissecting four typical reservoirs. The results show that the oil and gas accumulated in H3 reservoir come from the source rocks in Pinghu Formation; the reservoir and caprocks matched each other excellently, and the key factors to control oil and gas accumulation are the effective migration system and fault sealing, and the reservoir preservation condition has great influence on hydrocarbon accumulation.