Abstract: To diagnose the leakage fault of a cold gas thruster during the satellite’s assembly test,the leak rate of the thruster is retested by pressure vacuum method.Test results indicate that the leak rate of the thruster exceeds the required value.A penetrating scratch inside the contact notch of the non-metallic polytetrafluoroethylene(PTFE)sealing surface of the thruster valve is found through morphology analysis after disassembling the thruster.Dimensional measurements of the scratch channel by scanning electron microscope show that the length of the scratch channel is 220μm and the diameter is 13.7μm.Calculated leak rate caused by the scratch channel using molecular flow theory is 5.1×10^-5Pa·m³/s,which is consistent with the value measured by pressure vacuum method.Three typical valve leak modes and their morphological characteristics are summarized,including sealing surface plugged by residues,penetrating scratch in sealing surface and deflection of valve plug.Accordingly,methods for detection and improvement are proposed.