Abstract: The leakage of a membrane box assembly occurred during the pressure cycle test. The leakage causes of the membrane box assembly were analyzed by means of macroscopic observation, scanning electron microscope analysis, chemical composition analysis, metallographic examination and hardness test. The results show that the leakage of the membrane box assembly was caused by the cracking of the third diaphragm at the pressure end, and the cracking property was fatigue cracking. There were two scratch marks on the inner surface of the diaphragm, which caused local stress concentration and produce microcracks. Under the alternating stress of the pressure cycle test, the cracks continued to expand, and finally led to fatigue cracking of the diaphragm. There was a slight carburization phenomenon on the surface of the film, which accelerated the formation and expansion of cracks.