Abstract: With the continuous improvement of environmental protection pressure,in-depth denitrification retrofit for many fluidized circulating bed( CFB) boilers needs to be carried out to achieve ultra-low emissions,while there is little research on the application of SCR retrofit in CFB boilers at present. SCR denitrification system has been added to a 320 MW fluidized bed unit. Based on this project,the grid method was used to measure the parameters of flue gas temperature,flue gas composition,etc.,the boiler performance test before and after the retrofit was carried out,and the denitrification performance and its influencing factors after SCR retrofit were studied. Besides,the boiler efficiency was measured. The results show that the average temperature at SCR inlet is 268.11-309.53 ℃ under different loads,which basically meets the extended temperature window for SCR reaction( 260-420 ℃) . With 320 MW load,the measured denitrification efficiency is 72.48%,and the corresponding ammonia escape concentration is 0.7 mg/Nm3. Under 40%-100% load,the NOxemission is lower than 25 mg/Nm3,and the ammonia escape concentration is lower than 1 mg/Nm3. Because the flue gas temperature is lower than that of pulverized coal boiler,the denitrification efficiency of SCR reactor in this test is lower than that used in pulverized coal boiler. The urea consumption under 40%-100% load is lower than that of pulverized boiler with the same grade,and the urea consumption under full load is 279.09 kg/h. Under the same emission value,the urea consumption is reduced by more than 50%,and the effect of energy saving is significant. The results of emission and urea consumption show that SNCR are well coupled with SCR,and this technology has a great advantage in CFB boiler. The NOxdistribution at the SCR outlet is not uniform,showing a trend of high NOxconcentration on the right side and low NOxconcentration on the left side on the horizontal section of flue,which is consistent with the temperature distribution at the SCR inlet. Temperature is the main factor affecting the denitrification efficiency and NOxdistribution. The average boiler efficiency after retrofit is 90.07%,which is basically equal to that before retrofit,indicating that SCR retrofit has little effect on boiler efficiency. More than 90%of the heat loss of the boiler is caused by the heat loss of exhaust gas and incomplete combustion. The key to improve the boiler thermal efficiency is to reduce q2( heat loss of exhaust gas) and q4( incomplete combustion of solid).