Abstract: The Xihu Sag of the East China Sea is a petroliferous unit that developed in Cenozoic and is three-order Structure unit of the Esat China Sea Shelf Basin. Since the latest of Cretaceous, the Xihu Sag had been undergoing rifting Stage during the Paleocene and the Eocene, inversion Stage during the Oligocene and the Miocene, and regional subsidence stage during the Pliocene and the Quaternary. The sag exists a lot of Closures of structure, classified three types:fault block, pressed antic line and fold-thrust structure, which provided for the basis of the formation of oil and gas traps.