Abstract: Selective non-catalytic reduction（SNCR）is widely used in flue gas denitration technology of circulating fluidized bed（CFB）boiler. Generally,under 50%～100% load conditions,SNCR can achieve ultra-low NO_xemission requirements of boilers. The NO_xemission concentration of a 350 MW CFB boiler under different load conditions was tested and analyzed. When the unit load was 175 MW,the inlet temperature of cyclone separator was about 750 ℃. When the unit load is 105 MW,the inlet temperature of the cyclone separator is only about 640 ℃，and the reaction efficiency of SNCR denitrification is greatly reduced,NO_xemission and ammonia escape rate exceed the standard. In order to achieve the standard emission of NO_xwhen the boiler is under 50%of the load,the denitrification scheme is designed and transformed,and the transformation scheme is determined as flue Gas Recirculation+SNCR-SCR denitrification technology. The transformation is implemented in two steps according to the maintenance period, and the deep peak shaving test is carried out when the SCR selling system transformation is completed in the first step. The deep peak shaving test after the transformation shows that the denitrification efficiency under 100%,75% and 50% load is increased by 4. 27%,2. 65% and5. 70% respectively compared with that before modification. The denitrification efficiency under 30% load is more than 50% higher than that before transformation. After the transformation,the boiler NO_xemission under30%～50% load is up to standard,ammonia escape rate is greatly reduced,and the ash blocking problem of the air preheater before the transformation is alleviated. The standard emission of NO_xunder 30%～100% load of the unit can be achieved by the denitration scheme,which can provide guarantee for the unit to participate in the deep peak shaving, and realize great economic and social benefits for the enterprise..