The future manned mission for lunar and Mars needs minimizing the dependence on re-supply from earth. The Regenerative Life support System (RLSS) in NASA JSC is one part of this project. By using higher plants photosynthesis effect and transpiration, absorbs carbon dioxde and produces oxygen, purifies water and supplies food for astronauts. Since the difference of natural conditions between them and earth, such as low air pressure and microgravity, long day and night cycle, extremely temperature difference between daytime and nighttime, it is difficult to predict their influence on growth of plants. This paper focus on the effect of reduced pressure on Thermal Humidity Control subsystem (THC) and air distribution in Variable Pressure Growth Chamber (VPGC), and finally give some suggestions.