Abstract: In order to support an adequate residence for the crew, the key air environment parameters of the manned spacecraft should be controlled within an index range with the environmental and thermal control system, which involves a number of design and operational parameters. With the professional knowledge and the modeling language of Modelica combined, a simulation model for the manned spacecraft environmental and thermal control system is developed. With this simulation model, the relation between the air environment parameters and the crew metabolic level is studied from the system level with different speed ranges of the temperature and humidity control fan. According to the results, it is shown that in the case of other parameters kept unchanged, the greater the temperature and humidity control fan's speed is and the lower the air temperature is, the higher the relative humidity will be; the crew metabolic level could influence the air environment parameters dramatically. The air environment parameters could be maintained within the index range though regulating the control system operational parameters. Furthermore, the air environment parameters could not be analyzed separately due to the non-negligible effects of the air temperature on the oxygen partial pressure, the carbon dioxide partial pressure and the relative humidity.