The external curing method is usually adopted as the traditional molding process of thermoset fiber composite shell, which places the wound shell into the thermal compression reactor or the oven to cure. This kind of process greatly limits the molding efficiency because winding and curing proceed respectively and separately. The internal curing can be adopted because of the hollow structure of the cylinder-shaped shell, which heats the mandrel with high pressure steam to realize the in-situ curing of the wound shell. This paper researches the new fiber composite molding process named heated mandrel winding. Not only does the heated mandrel winding inherit the advantages of internal curing process, it also greatly improves molding efficiency and quality. This paper introduces the process principle of heated mandrel winding and uses the finite element analysis software FLUENT to carry out the numerical simulation of steam flow and heat transfer during the mandrel heating process, and the simulation results are verified by experiment. According to the simulation results, the distribution and changing course of the flow field and temperature field of the system are obtained, and the relationships between the physical quantities including pressure, temperature and velocity and the parameters of the steam control and channel structure are analyzed.