Abstract: T he a ppearance of fossil Cyprideis was demarcated on the magnetic biostratigraphic column in Qaidam Basin. T he eruptible time of Cyprideis is at the later Middle Miocene, a bout 12Ma ago, which is the important evidence to divide the boundary between Shangyoushashan Formation and Xiayoushashan Formation, and also key mark that seismic T2 reflect interface is used to trace and validate whole basin. The eruptible point of Cyprideis can be recognized clearly in Tarimand Junggar Basins, which has been pla yed a n importa nt role in the stra tum classification and contra sting of Neogene inside in these two basins as well as comparing with that in Qa idam Basin. It is suggested that the boundary between Shangyoushashan and Xiayoushashan Formation would be determined basica lly through sampling few core and much debris a nd drawing the abundant curve of Cyprideis existed in debris.

Key words: ostracoda, eruption point of biology, stratigraphic correlation, curve of fossil abunda nce, debris log in drilling well, Qa idam Basin