Abstract: A low exploration success ratio on the Baochu Slope of Xihu Sag of East China Sea has been inferred, and the ratio in the Central Anticline Belt varies significantly in different sub structures. To solve this issue, the petroleum geologic conditions of the sag were systematically reviewed in light of new basin analysis methodologies, and new understanding have been achieved. Due to the differences in tectonic features and evolution histories, the Baochu Slope and Central Anticline Belt developed different petroleum systems and play types. Only those areas with overlapping favorable fairways of all petroleum system elements promise the formation of large oil and gas fields. The Baochu Slope is featured by tectonic rifting, with weak later compression and early structure formation. Its favorable conditions for hydrocarbon accumulation include: short distance to the main source kitchen; multiple, overlapping, large-scale reservoirs; multiple structural-stratigraphic trap types; good vertical sealing zones; and effective lattice pattern migration pathways. The large to medium structural-stratigraphic fields are concentrated in fairways with a large scale source kitchen and updip pinchout of tide-dominated delta sands controlled by paleo slope break which in turn were induced by either faulting or paleo morphology. The Central Anticline Belt has experienced strong inversion and compression during the late depression stage, and developed inverted compressional slip faults and large scale anticlines. Hydrocarbon accumulation is controlled by four major factors: preexisting structural highs before the compressional event, large scale structural-lithological traps, excellent timing of hydrocarbon charging during deformation, and highly effective reservoir-seal combinations. The areas with good congruence of these contro-lling factors promise the occurrence of large-scale deposits such Longjing, Guzhenzhu, Huxinting fields.