In the space station, flexible water tank products are water storage containers for environmental control and life support. The flexible water tank products are responsible for the collection, storage, transportation, and management of various space fluids. Including astronauts’ drinking water, recycled water, urine distilled water, condensed water, pretreated urine, and pretreated urine residue after distillation. Therefore, the sealing performance of flexible water tank products directly affects the reliability of the environmental control and life support system in space station. In this paper, pressure cycle tests and long-term negative pressure holding tests are carried out on flexible water tank products. The results show that after the pressure cycle test and the long-term negative pressure holding test, the sealing performance and leakage rate of the nine-layer composite membrane water tank product do not decrease significantly, which has certain guiding significance for the sealing design of this kind of products in future.