Abstract: With the increasing requirements of environmental protection,nitrogen oxides( NO_x) have become the focus of gas pollutant emission control after dust removal and desulfurization in thermal power plants.The corresponding boiler combustion system should also be changed when the existing units are reformed for ultra-low emission.The change of combustion system has a direct impact on the temperature field in the furnace.The formation and shedding of oxide scales in furnace tubes has been the main treatment problem of ultra-supercritical units,while the oxide scales of sub-critical units have not been effectively concerned.The reformation of low nitrogen burner of boiler mainly uses four-zone theory,namely pyrolysis zone,main combustion zone,reduction zone and burnout zone,to delay combustion by reducing excess air coefficient in the initial stage,which has a certain impact on the temperature field in the furnace.At this time,the change of burner will aggravate the change of temperature field in the furnace and stimulate the sensitivity of tubes with lower oxidation margin in the furnace.In this paper,several tube bursts occurred continuously after the retrofit of 330 MW subcritical unit with ultra-low emission in a power plant.By comparing the causes of tube bursts,the aging of tubes and the changes of temperature field in the furnace before and after the retrofitting,it is confirmed that the main cause of tube burst caused by the change of temperature field in the furnace is the low oxidation margin.In view of the secondary problems that may arise from the retrofitting of subcritical boilers,it is proposed that attention should be paid to the change of temperature field in the furnace after the retrofitting of the burner during the retrofitting of ultra-low emission boilers.When the temperature field of SA213-T23 in the subcritical unit rises by about 20 ℃,the acceleration of oxide scale production and the acceleration of aging of metal structure are prone to occur.The subcritical unit should strengthen the detection of oxide scale,grasp the aging degree of pipes,and expand the antioxidant margin of pipes appropriately in material selection.