The design and manufacture of high efficiency and energy-saving heating and gasification equipment is an urgent problem to be solved in the oil and gas industry. In-depth research has been carried on the heat transfer mechanism inside the heaters. A combined natural convection and participating medium radiation heat transfer model in the large cylinder was built in a creative way, to solve the unsteady-state coupled heat flow field in the gas heater with water as the heat transfer medium, and the boundary conditions and numerical model were validated through the gas heating and heat transfer flow test platform. The results show that the flow field in the heater is uneven and the overall circulation is poor. The starting time of the heater is 2.5h, the heating efficiency is only 83.5%, and the medium participatory radiation accounts for 24.5% of the total heat transfer. Obviously, the structure of traditional heater needs to be optimized, and the influence of medium participatory radiation can not be ignored.