In order to develop a water vapor measurement and dewatering device suitable for ground test of spacecraft sealed cabin. A moisture measuring device suitable for the sealed cabin was designed. The existing moisture content measuring sensors were equipped with auxiliary devices to protect the electrical circuit of the sensor, and then the sensor was calibrated and analyzed. After calibration, it can meet the requirement of moisture measurement in vacuum environment. A small chamber was built to study water change rule in the pumping process. In the water removal system, the cabin was connected to the vacuum pump system through a two-way sealing flange, which can complete the water removal outside the vacuum chamber by pumping. When the pressure of the sealed cabin reached 100 Pa, high purity nitrogen gas was added to 97 kPa, then pumped in again. The results show that the water removal method can ensure that the dew point temperature of the sealed chamber is lower than 0°C during the vacuum thermal test. After protected, the dew point sensor can measure moisture at low pressure. The water removal system can effectively remove the moisture in the sealed cabin, thus ensuring the completion of the vacuum thermal test.