The invention provides a flow mixing device, which belongs to the technical field of SCR denitration of a coal-fired boiler of a thermal power plant, The flow mixing device comprises a swing flow deflector, comprising a first flow deflector and a second flow deflector which are arranged on two sides of a flue center line as well as are respectively and rotatably connected with a flue side wall, aflue is divided into several sub-flues by the first flow deflector and the second flow deflector; and an angle adjusting mechanism is respectively connected to the first flow deflector and the secondflow deflector for driving the first flow deflector or the second flow deflector to rotate. The invention also provides a SCR denitration system and a denitration system flue flow field homogenizationmethod. The flow mixing device, the SCR denitration system and the SCR denitration system flue flow field homogenization method solve the technical problem that the installation and fixing of the flow deflectors in the flue flow field cannot guarantee the uniformity of the flow field in the prior art.