Objective The efficient gasification of liquefied natural gas LNG is a key influencing factor in the industrial application of natural gas. In order to improve the operation efficiency of the existing gasification equipment in the context of national energy reform ANSYS Fluent software was used to numerically simulate the unsteady coupled heat flow field in a natural gas gasifier using ethylene glycol as the intermediate heat-carrying medium. Methods The heat and mass transfer mechanism of natural gas gasifiers was studied in depth. It was found that previous studies only considered convective heat transfer. Considering the radiative heat transfer a coupled heat transfer model of unsteady natural convection and medium participating radiation in a large space was constructed. The boundary conditions were verified and calibrated by natural gas heating and heat transfer flow experimental equipment. The correctness of the numerical analysis model was verified. Results When the running time of the gasifier with ethylene glycol as the intermediate heat-carrying medium reached 2. 0 h the heat transfer tended to be stable. As the internal thermal flow field was no longer obviously disturbed the gasifier entered a stable operation stage. At this time the overall heating efficiency in the furnace was only 87. 35% in which the medium participating radiation accounted for 27. 01% of the total heat transfer. In addition a small range of low-temperature zones was formed at the bottom of the gasifier indicating that there was a flow dead angle that was difficult to eliminate in its internal operation under this working condition. Conclusion Under the working conditions set in this paper the internal flow field distribution of the large cylinder natural gas gasifier is not good and the heating efficiency and start-up time of the gasifier need to be further optimized. The main form of heat transfer in the furnace is still natural convection but the contribution of medium participating radiation to the total heat transfer cannot be ignored.