Abstract: With the improvement of the manned space technology in China, a large space environment simulation test facility, the KM6 horizontal chamber, is under development, in which integrated tests of man, suit, and vehicle can be conducted. This paper will describe the environment control system (ECS) of the test facility. According to the requirement of the proposed tests, the ECS should firstly supply desired gases, namely, oxygen, nitrogen, or the oxygen-nitrogen mixture, for the test crew and the test vehicle in the chamber, and then take charge of the environment control for the test chamber and the test space vehicle under specific circumstances. For the function of supplying gases, the ECS should consider the emergency oxygen supply for the test crew inside the test chamber in case that the backpack life support system fails, besides the normal air supply according to the proposed ends. The function of the environment control covers the control of various parameters of the chamber environment such as temperature, humidity, chamber pressure, oxygen partial pressure, and the removal of the gaseous contaminant. Not only one has to pay attentions to the stable control of each parameter, but also one should put special emphasis on the overall control of all above parameters during the depressurization of the test chamber because these parameters are closely coupled. In order to study the relationship between them, a mathematical model for the controlled chamber is built up. From results obtained under different boundary conditions, the optimum control scheme is mapped out.