SCR denitration system is the use of catalyst, under a certain temperature (320 ~ 420 ℃ or so) the NOx in flue gas with ammonia reduction reaction in the aftermath of the supply systems for mixing injection of NH3, generate N2 and H2O, thereby reducing NOx emissions, reduce the pollution of the flue gas to the environment. The technique, with no by-products, does not generate secondary pollution. The device structure is simple, and the removal efficiency can reach more than 80%, and the environment temperature is low, the flue gas temperature can meet the needs of the catalyst operation, ammonia escape 3 PPM or less, of SO2 / SO3 conversion rate of 1% or less, is applicable to any operating conditions.