Abstract: In order to solve the problems of water wall breakage, large heat loss and high NO_xemission in low load operation of a 330 MW W flame boiler, and to ensure the performance of this type of unit in flexible operation, the original design parameters of boiler combustion system were compared with those of similar units, and the reconstruction scheme of combustion system was put forward and implemented on site. The results showed that the low original design value of furnace section heat load and the unbalanced input of furnace heat load were the main reasons for the cracking of water wall tube; Only a small amount of secondary air entering the furnace from the arch could not reach the injection function of the primary air pulverized coal flow, which could not guarantee the penetrability and rigidity of the primary air; Incident position and angle of secondary air under arch were unfavorable to ignition and burnout; The degree of staged combustion of air on the whole furnace was very poor, there was no independent OFA nozzle, and the control ability of NO_xgeneration was weak. The proposed renovation scheme mainly included: adding secondary air nozzle on the arch, placing the exhaust gas nozzle on the back fire side of the concentrated pulverized coal airflow, and reforming the cold sleeve of OFA nozzle on the arch. The field test after the transformation showed that the combustibles of fly ash were reduced by about 0.95 %, the NO_xconcentration AB side before denitration was respectively reduced by 140 mg/m~3and 129 mg/m~3compared with that before transformation under the 210 MW load condition.