Vuggs，caves and fractures are developed in the Ordovician reservoirs in Halahatang Oilfield，and these complex oil and gas reservoirs are composed of the fracture-cavity units superimposed on each other with different pressure systems and oil-water relationships. These reservoirs are characterized by the coexistence of the multiple pressure systems，oil-water relationships，and development performances. As a result，the intricate oil-water relationship increases the difficulty of development and results in a low drilling success rate. At the same time，due to their particularity，these reservoirs have massive vented and lost sections. It is difficult to identify the original oil-water interface distribution and change law by traditional direct prediction or monitoring methods，and the reliability is low. In view of this，the static volume of the reservoir is calculated by the geological modeling method based on the reservoir’s uncertainties. The dynamic oil volume and water volume are respectively calculated by the reservoir production instability analysis method. When the volumes of dynamic and static reservoirs are the same，the original oil-water interface is gradually adjusted to ensure that oil and water volumes characterized by the static model are consistent with that by dynamic evaluation，so as to determine the original oil-water interface of the well. In addition，by applying the method not only in one well or certain areas but also in the whole region，the paper can comprehensively correct the oil-water interface position with information such as geological laws，as well as production and drilling tests. The new method provides new ideas for predicting the original oil-water interface of fracture-cavity reservoirs and reserving water avoidance height，and it lays a foundation for further adjusting development measures of the oilfield.Measures such as infilling well pattern，sidetracking and gas injection can be used to tap the potential.