A new passive check valve for Mechanical Pumped Cooling Loops (MPCL) is designed. Static structural analysis and flow characteristic simulation of the diaphragm-type passive check valve designed are performed by the software ANSYS. The forward flow and leak flow characteristics of the check valve are tested experimentally. The results show that the performance of the check valve can completely meet the requirements of the mechanical pump. The check valve is of referential significance to the research on the improvement of the performance of mechanical pumps.