Abstract: Analyzed in the essay is the outer surface crack of the large-sized,heavy-wall P91 steel tube.It is identified via observation of the macro and micro morphologies of the crack that the crack is free of any oxidation/decarbonization and inclusion,and the fracture is of the brittle fracture,which is in conformance with the characteristic of the quench-caused cracking.Based on relevant on-the-site investigation,it is regarded that the crack is of a quench-caused type as formed due to uneven cooling during the heat treatment process.Accordingly actions aiming at improving the cooling conditions for the steel tube during the quenching operation as below need to be taken to have the steel tube avoiding uneven cooling during the heat treatment operation so as to completely stop the said quench-caused crack.Such actions include cleaning the primary scale in the quenching tank to get the tubes avoid touching to each other during the quench operation,and strengthening the circulation of the water in the tank or adopting the external showing+internal spraying tube-rotation device to cool the tube,etc.