In order to reduce the heat resistance and improve stability of convective heat transfer in liquefied natural gas(LNG) vaporizer, the flow and heat transfer processes of supercritical pressure LNG were analyzed by numerical method in a vertical tube. The influence of heat fluxes and flow directions on heat transfer characteristics was discussed, and the effects of the flow field , the temperature field , and the turbulent kinetic energy changes on the heat transfer instability were investigated.The results show that the heat transfer of supercritical methane in vertical tube is unstable,at high heat flux density, the inner wall temperature and the average temperature of the pipe are unstable and and shock in the heat transfer deterioration range; At the low heat flow density , the local convection heat transfer coefficient is unstable , and it shocks in the heat transfer deterioration range; Under the same heat flux density, the heat transfer instability of upward flow is greater than that of downward flow, because the thermal influence of upward flow is greater than that of downward flow. The gas?like film is the main reason of heat transfer instability.