Abstract: The numerical simulation for the design of the hot gas mixing structure of Pebble-bed Module High Temperature gas-cooled Reactor（HTR-PM） are carried out with consideration of the leakage flow out of the reactor core. According to the profiles of temperature, pressure drop and flow velocity, the thermal mixing is mainly produced by the secondary flow such as the whirlpools which is perpendicular or parallel to the main flow direction. In addition, the pressure drop is mainly caused by the pressure loss leading by the sudden change of flow area and direction which is also the main reason of production of secondary flow. The numerical simulation indicates that the design of the hot gas mixing structure at HTR-PM reactor outlet can fulfill the requirement of high thermal mixing performance and low pressure drop by taking into account the leakage flow out of reactor with the condition of rated flow.