A failure analysis was conducted for the leakage of metal diaphragm in expulsion efficiency test of propellant tank by the methods such as macrostructure and microstructure analysis，EDS and metallographic analysis.Through the simulation of the metal diaphragm rolling process，the stress and strain behavior was studied.The results show that the major cause of the leakage is the gas porosity in diaphragm.The gas porosity diameter（about 0.45 mm） nearly penetrates the material wall thickness（about 0.45 mm），causing a reduction in the load-bearing capacity of the leakage area.Before the end of expulsion efficiency test，the material wall thickness reduction caused by diaphragm rolling is less than 0.1 mm.At this moment，the porosity defect has not leaked.At the end of expulsion efficiency test，the diaphragm is about to adhere to the inner wall of propellant tank，which increases the pressure difference on the diaphragm to 1.7 MPa.And 300 MPa stress and 0.2 stain exceeds the defect exposure limit.The gas porosity ruptured，which resulted in diaphragm leakage generated finally.