Abstract: The reason of low pressure explosion of TC4 titanium tank equatorial weld was investigated by macroscopic examination, fracture analysis and metallographic examination. The results show that during the welding, the weld center gradually deviates from the joint junction face, the molten pool boundary at the top weld position is close to or even coincide with the joint junction face, then partially molten zone with low strength and toughness and many tiny plane-distributed pores are formed, which reduces the effective cross-section of the weld and intensifies stress concentration. During the process of pressure test, the crack source is located at partially molten zone and pores, and the brittle fracture is formed, which causes low pressure explosion of the tank. Finally, according to the analysis results,corresponding suggestions for improvement were put forward.