Abstract: A hot gas mixing structure was designed under the reactor core of high-temperature reactor-pebble-bed module (HTR-PM) to get sufficient thermal mixing of helium coolant which comes from the reactor core. A hot gas mixing experiment of outlet of HTR-PM reactor core was proposed to measure and analyze the actual mixing performance and resistance property of this mixing structure. By considering economic cost of the experiment, the design criteria and parameters of the hot gas mixing experiment were determined according to similarity criterion. In addition, the numerical simulations of the temperature and pressure profile of the designed experiment installation were carried out by using Fluent software. According to the theoretical analysis and the numerical simulation, it is concluded that there are similarity on thermal mixing and flow between the experiment installation and the actual mixing structure of HT-PM. Based on the experiment, the mixing performance and resistance property can be obtained through theoretical analysis and numerical simulation.