Abstract: In order to secure a healthy environment for astronauts and eliminate the dews in the sealed cabin, a set of issues on the humidity control in a sealed cabin of a manned spacecraft are studied: 1) the evaluation model of the relative humidity for the dry air replacement in the manned spacecraft before the hatch is closed on the ground; 2) the dehumidification by condensing the heat exchanger of the spacecraft in orbit. The balance equation of the humidity is established, and the flux of condensing the heat exchanger is obtained. The evaluation model of the relative humidity and the ventilation can be a crucial problem in the actual engineering design.