Abstract: Inner surface defects like pit and micro-crack are found with the Φ457 mm×20.8 mm 34CrMo4 hot-rolled gas cylinder pipe after being hot-expanded，cold drawn，and inner surface ground and polished. And the original factors of the said defects are revealed by means of taking chemical composition analysis，micro and macro inspections，metallographic analysis，and energy spectrum analysis of the defective pipe sample. It is regarded via the analyses that the defect shapes are different and distributed in random；beside the iron oxide，the pipe contains a lot of chemical elements like Ca，Mg and Si etc.，and there is slag inclusion in the inner surface defective part；considering the decarbonization phenomenon existing around the defect and there are oxidized pots，it is determined that the said slag inclusion is originated from the deoxidation products as developed during the steel-making process. Accordngly it is suggested to strictly control the melting and casting process parameters during the making process of the steel billet so as to ensure the billet cleanness and improve the billet quality.