When a serious accident occurs in a nuclear power plant, the emission of high-temperature working fluid will produce a heating and boosting effect, thus destroying the integrity of the containment. At present, the Passive Containment Heat Removal System (PCS) is widely used in the world to reduce the pressure and temperature in the containment. A large amount of steam released into the gas space transfers the heat to the PCS heat exchanger in the form of condensation, and then transfers the heat to the atmospheric environment through the natural circulation flow of cooling water. However, when the accident occurs, the steam in the gas space is often not fixed due to the different flow directions of the break position, and the forward, reverse and even cross erosion may occur. In this paper, the condensation process of steam with non-condensable gas outside the vertical smooth tube is numerically simulated by constructing a diffusion boundary layer condensation model and using CFD method. The research focuses on the influence of three flow directions of forward flow, reverse flow and cross flow on the condensation heat transfer characteristics outside the tube under the conditions of pressure of 0.32 MPa, air fraction of 0.57, different subcooling and flow velocity. The simulation results show that the increase of flow velocity significantly improves the condensation heat transfer coefficient of cross flow, but has little effect on forward flow and reverse flow. With the increase of subcooling degree, the condensation heat transfer coefficient will decrease, because the increase of subcooling leads to an increase in the thickness of the high-concentration air layer, which in turn increases the thermal resistance. In general, the enhancement effect of transverse erosion is the most obvious, which provides a reference value for the structural design optimization of heat exchanger.