Through the collaboration between Qinghai Oilfield Company and China University of Petroleum(Beijing), ostracoda Austrocypris fossil zone was found in the interval about 5120 m to 5140 m in Well Kun2, Qaidam Basin. By correlating with the  Austrocypris fossil zone in the western and northern margins of the Qaidam Basin, its top boundary was calibrated with a seismic event and traced on the seismic profile. The results indicate that the Austrocypris is an index fossil of the upper part of Lower Member of Xiaganchaigou Formation in the Qaidam Basin. Tracing along the seismic lines from Well Lengke1(or Shen88) in which Austrocypris was found and T4 seismic reflector surface was marked, to Well Kun2, after time to depth coversion the boundary between the upper and lower members of Xiaganchaigou Formation is determined to be at about 5040 m in Well Kun2. The T4 seismic reflector surface, the top boundary of Austrocypris fossil zone and the T5 seismic reflector surface（at the bottom of Xiaganchaigou Formation） are always parallel with one another between the two wells. The boundary between the upper and lower members of Xiaganchaigou Formation is reasonable and its position is supported by the loop correlation of seismic events. In conclusion, combining biostratigraphy with seismic stratigraphy, an effective correlation of the Cenozoic stratigraphic succession of  the red continental clastic rocks in the Qaidam Basin has been obtained.