The invention discloses an SCR denitration system capable of being put into operation under the working condition of a low load of a boiler. The SCR denitration system capable of being put into operation under the working condition of the low load of the boiler comprises a primary air fan, an air preheater, an ammonia supply system and a SCR denitration reactor, wherein the primary air fan is connected with the air preheater through a primary air pipe, the air preheater is connected with the ammonia supply system through a hot primary air pipe, and the ammonia supply system is connected with the SCR denitration reactor through a mixed air pipe; and the hot primary air pipe passes through the surrounding area of a boiler combustor, and the hot primary air pipe between the air preheater andthe boiler combustor is connected with a diluted air fan through a diluted air pipe. According to the SCR denitration system capable of being put into operation under the working condition of the lowload of the boiler, a hot primary system at an outlet of the air preheater is transformed, and the hot primary air pipe is connected into the surrounding of the combustor in the boiler, so that secondary heating for hot primary air is realized; and even if the boiler is just started, effective heating for the primary air can still be realized through the combustor, so that preheating for the SCR reactor is realized, and the SCR reactor is put into operation under a lower load.