Abstract: In view of plate movement and hydrodynamic conditions, a mechanism of forming ancient karst land-form of Ordovician in Tahe oilfield is discussed. It is pointed out that the most favorable developed zones of pores, caves and fractures are firstly the steep coast and concave coast of solution channel as well as gentle slope zone of higher relief; secondly the monadnocks of higher relief; finally the zones such as pothole surroundings, dissolution potholes and slope area. The important factors for forming ancient karst landform in Tahe area are powered as plate movement, back grounded as uplift，conditioned as faulting and water, of which the key is timing, while foot cave is the foundation of forming big cavity-developed zones.

Key words: Tarim basin, Tahe oilfield, Ordovician, ancient karst, karst feature, forming factor, plate movement, fault