Abstract: The flow field in the hot gas chamber of High-Temperature Gas-Cooled Reactor (HTGR) was studied with computational fluid dynamics program CFX5 on the basis of the experimental studies. The velocity, pressure and temperature fields in hot gas chamber and hot gas duct were obtained, which is helpful to the design and experimental study of hot gas chamber. The results indicate that there is much highly turbulent twisting flow to be responsible in excellent temperature mixing effect in hot gas chamber. But the flow in the ribs region is calmly, which hinders the heat transfer between the hot gas and cold gas. The temperature mixing coefficient increases with the increase of the hot gas duct’s length, and the temperature mixing coefficient reaches 99% when the length of hot gas duct reaches 2.5 m