Abstract: Water wall tube of circulating fluidized bed (CFB) boiler of a petrochemical enterprise leaked, causing shutdown failure of boiler.The failure mechanism of CFB boiler water wall tube was studied via analysis of macro-observation, chemical composition, mechanical properties, metallographic structure, fracture morphology and EDS, combining with field process parameters as well as temperature simulation.The results show that the main failure of water wall pipe of CFB boiler is that the leaked location has repaired weld and there is welding hot crack-polygonal crack at the leaked location, which propagates until the penetration leakage occurs under inner pressure and differential temperature stress.Decrease of toughness after long-period service and alkali concerntration are attributed to the acceleration of crack propagation.