With the improvement of the manned space technology in China, a larger man-rated space environment simulation test facility, KM6 Horizontal Chamber, is under development, in which the integrated test of man, suit, and vehicle can be conducted. This paper will describe the Environment Control System of the test facility. According to the requirement of the proposed test philosophy, the ECS should firstly fulfill the function of supplying desired gases, namely oxygen, nitrogen, or the oxygen-nitrogen mixture, for the test staffs and the tested vehicle in the chamber, and then be in charge of the environment control of the test chamber and the tested space vehicle under specific circumstances. As for the function of gas supply, the ECS takes into account the emergency oxygen supply for the tested crew inside the test chamber in case the backpack life support system failed, besides the normal air supply to the proposed ends. As for the function of environment control, the ECS covers the control of such aspects of the chamber environment as temperature, humidity, chamber pressure, oxygen partial pressure, and the removal of the gaseous contaminant. In spite that sufficient consideration has been taken on the stable status control of each coefficient, special emphasis has been addressed on the comprehensive control of all the above aspects during the depressurization of the man-rated test chamber because the coefficients mentioned above are closely coupled between one another. In order to investigate the relationship between the coefficients, a mathematic model of the controlled chamber is built up. From results achieved under different boundary conditions, the optimum control scheme is drawn out.