Abstract: LNG ambient air vaporizer (AAV) is the key equipment in LNG vaporization stations, so its vaporization and heat transfer performance inside the pipe is of great significance to its design and operation optimization. In this paper, the single finned tube of AAV in service was taken as the model for study. The vaporization, heat transfer performance and flow pattern of LNG inside the pipe were numerically investigated in Volume of Fluid (VOF) multiphase flow model and Lee condensation phase change model by using the fluid mechanics calculation software Fluent. It is shown that the vaporization flow patterns of LNG in AAV are mainly bubbly flow, slug flow, churn flow and mist flow, which are in line with Hewitt and Roberts flow pattern maps. During the vaporization of LNG inside the pipe, the local heat transfer coefficient increases firstly and then decreases as the gas volume fraction gets higher. The local heat transfer coefficient inside the pipe increases with the increasing of heating flow density at the wall surface. The vaporization and heat transfer of LNG in AAV is dominantly in the form of nucleate boiling.