Abstract: The development on Ordovician carbonate reservoir of Tahe has been carried out for 10 years but the variance regu-larity on its oil-water contact hasn't been reached,which brings lots of confusion for reservoir development. Starting from the study on fracture-cavity characteristics and flow-ing status,the cave systems,the distribution characteristics of the karst caves,tiny holes and flowing status have been considered to study the oil-water contact variance in the pa-per.① the typical characteristics of pile flow in caves: sur-face contact is oil-water contact; seepage in tiny fracture -cavity and the oil-water contact is coning in dot scope and there is oil-water transitional zone; ② the breakthrough in caves is caused by pushed bottom water,which could present the depth of oil-water contact and time of water-pushing,while in tiny fracture-cavity,the breakthrough doesn't stand for oil-water contact variance; ③ edge of water in reservoir development process is pushed forward to the top of the cave system step by step,under certain conditions,coning to branch caves and tiny fracture-cavity successive-ly,forming different oil-water contact in different parts of body and showing different recovery degree.