Abstract: Pressure vessels are an important part of the tank pressurization system of carrier rockets, welded with pressurization pipes using all position welding technology. The welding joint structure is the O-shape collar with inner bladder. In the process of welding, the number of welding blowholes at the welding joint of vessels and pressurized piping exceeds the standard requirement. The morphology and distribution characteristics of blowholes were analyzed, and the formation cause for the blowholes was analyzed by spectrum and infrared spectrum analysis. It is found that the residual grease reminders are the main cause for the blowholes. Recurrence of the blowholes was carried out by air tightness and all position welding testing to verify the judgment. According to the formation cause for the welding blowholes, some measures were taken to effectively prevent the formation of welding blowholes: repeatedly scrubbing with silk cloth filled with alcohol solvent and adding an endoscope inspection process.