Abstract: To get the fluid flow characteristics in the flow passage of the subsea christmas tree body, aiming at the 1500 m subsea christmas tree of a subsea gas field in South China Sea, the models for christmas tree body, tubing hanger and internal flow channel are established. Meanwhile, the steady and unsteady coupled heat transfer simulation analysis of fluid flow field and temperature field are carried out by Fluent. The results show that when the fluid flows in the T-shaped channel of the christmas tree body, the pressure distribution in the channel is uniform, the velocity becomes faster at the upper corner and slower at the lower corner, and the temperature of the blind pipe section is lower than that in the flow area. At the same time, inlet temperature, inlet flow velocity, seawater temperature and T-tube size all affect the flow and heat transfer. Finally, by adding insulation layer on the basis of the original model, the unsteady coupling heat transfer analysis of underwater christmas tree is carried out, the decline curve of fluid temperature in 8 hours under well shutdown condition is obtained, and the method of calculating the optimal insulation layer thickness is determined.