Abstract: Flow units, which has made much progress in the study of fractured carbonate reservoir, has great practical significance for improving oil recovery ratio. As for the fracture - cavity carbonate reservoir, the conventional theory of flow units is hard to apply because of the complex influence of tectonic structure, post diagenesis, tectonic fracturing and dissolution. Based on the typical geological character of fracture - cavity carbonate reservoir in Tahe oil field, this article discussed the existence and variance of seepage barrier and brought forward the concept of flow units, which is quite different from that of fractured carbonate reservoir. The proposed research clue and methods included: pressure trend analysis, production interference analysis, fluid PVT differences, and interference well test. Combined with the reservoir features discussed above, we has established dividing criterion of fractured carbonate reservoir flowing unit.