Failure analysis was carried out to find out the causes of burst on water-cooled wall tubes elbow for circulating fluidized bed (CFB) boiler.Through macro-observations, chemical composition analysis, macro structure observations，grain size measurements and water quality examinations, it can conclude that the high pH value of boiler water at the fire-facing side of weld dense phase zone will lead to alkali concentration that will cause alkali corrosion. The existence of residual stress in elbow under strong alkaline environment will cause alkali stress corrosion. Research results show that the tube failure is caused by both alkali corrosion and alkali stress corrosion. Strengthen water quality monitoring and elimination of stress can effectively prevent the alkali corrosion and alkali stress corrosion.