Abstract: It’s a potential developing direction for power generation of high temperature gas-cooled reactor combined with helium turbine closed cycle. The high speed mechanical performance test plays a key role in design and development of helium turbine system. It would verify the mechanical performance of turbine-compressor which was supported by active magnetic bearings. The results would provide basis for normal operation while helium turbine system is successfully connected with reactor in the future. A high speed mechanical performance test of helium turbomachinery was designed and analyzed. The rotor was supposed to be arranged in an enclosed vessel and low density environment inside the vessel can be acquired. By analyzing the preliminary thermo-hydraulic characteristics of test system, turbomachinery performance under low density was achieved and temperature distribution was estimated. The results indicate that the maximum temperature of motor’s stator is the temperature limit of test system. The thermo-hydraulic analysis shows there exists a certain pressure adjusting range to achieve the safe operation of test and it ensures the feasibility of this low density plan.