Abstract: The migration system, as one of the key research targets in oil and gas exploration, has been studied in the Xihu Sag, the East China Sea Shelf Basin. Fault, sandstone and unconformity, the basic types of migration system in the Xihu Sag, always work together vertically and laterally to form the migration network for a hydrocarbon accumulation. According to the spatial and temporal relations of the basic elements of fault, sandstone and unconformity, 4 kinds of migration system are recognized in the Xihu Sag. They are the terrace-like migration system along a forward fault; dome-like migration system along a reversed fault, "Y"-type and "T"-type of migration systems along an adjustment fault. The migration system is a controlling factor on hydrocarbon accumulation. Firstly, it will decide the distribution patterns of hydrocarbon accumulation strata. Secondly, the geological factors of the migration system will control over the distribution pattern of hydrocarbon accumulations.