A high-temperature gas mixing chamber is a type of mechanical device which has been widely used to obtain airflow at a specific high temperature. When a temperature environment test for aviation equipment is performed, the airflow temperature should exceed 1, 000 K and change fast. However, the existing mixing devices cannot meet the requirements for mixing speed and uniformity of temperature environment tests. To solve the problem of low mixing speed and uniformity, this paper proposes an innovative gas mixing chamber design. The proposed design allows cold and hot gases to be injected in directions perpendicular to each other, which increases the collision and heat exchange of gases with different temperatures. In this way, a faster mixing process and a more uniform outlet temperature field are obtained. Moreover, a genetic optimization method is used to improve the performance of a mixing chamber. This method considers temperature distribution, the velocity distribution of the outlet airflow, and the mixing speed. Simulation results show that the proposed method can reduce the mixing time from 4. 4 s to 2. 3 s, the standard deviation of the outlet temperature distribution from 25. 2 to 14. 7, and the standard deviation of velocity distribution from 1. 3 to 0. 36. The experimental results show good consistency with the simulation results, indicating that the simulation and optimization results are reliable, and the mixing performance of the cold and hot gas mixing chamber is significantly improved by the proposed method. The proposed method is important for optimizing the design of similar orifice plates and flow mixing structures.