By means of detailed interpretation and variance cube surface slice technology of high frequency 3-dimentional seismic data, a great denseness of microfaults are firstly discovered in Miocene strata in Changchang Depression, Qiongdongnan Basin. They are approximately in SN direction, cut off 6~28 m and 0.5~4 km long with about 60° obliquity. It is indicated that these microfault systems neither havea consequential relationship with early faults nor polygonal faults that recently discussed on so they were formed by local extension in special geological conditions during Mesozoic period. The genesis of them is summarized in four aspects: 1) During Miocene, Qiongdongnan Basin had entranced postrift stage and Changchang Depression developed with no faults except for basin-controlled faults. 2) SN-directional microfaults would not develop in the depression because there is not EW-directional stress in this region. 3) The Oligocene faults and Miocene faults are two sets of fault systems because the Oligocene faults are in NW direction and have great difference in spatial distribution, which are dominantly controlled by Paleocene structural patterns that, while the Miocene microfaults are in SN direction and have no relationship with the Oligocene fault system. 4) Polygonal faults are commonly with diverse boundary numbers and anomalistic boundary shapes, which are greatly different with Miocene SN-directional faults. It is believed that the existence of these high dense microfaults is most significant to oil and gas exploration in deep water.