Abstract: In order to study the local characteristics of the air-water counter-current flow, a numerical simulation of interfacial wave in a vertical tube with 25 mm diameter has been carried out based on the VOF（volume of fluid） model. The change of interfacial wave was analyzed and showed a good agreement with the experimental film thickness. Through the analysis of the variation of the flow field under different liquid flow rates, it can be concluded that the pressure difference generated between the windward and leeward of the interfacial wave introduces an additional force, which weakens the stability of the interfacial wave. And the pressure difference increases with the increase of the liquid flow rate. Moreover, the greater the gas flow rate, the faster the amplitude of the interfacial wave develops along the axial direction, indicating that the instability of the interfacial wave increases with the increase of the gas flow rate. Consequently, the increase of the gas and liquid flow rate reduces the stability of the interfacial wave, namely, the flooding is more likely to occur.