Selective catalytic reduction (SCR) with ammonia is a key technology to reduce the NOx emissions from the coal-fired power stations. The denitration efficiency of SCR system depends on whether the flue gas and ammonia can get a good mixing in the reactor. Combined with the species transport model and porous medium model, the large eddy simulation (LES) is carried out to simulate the transient multi-species flow in an SCR system at a 310 MW coal-fired station. Predicted transient total pressure drop histories for various loads are compared against the measurements, and close agreements are achieved for the mean values. The instantaneous velocity field is composed of various small recirculation zones and multiple vortices, and their positions and intensity are ever-changing. For the SCR reactor without any gate leaves, a big secondary recirculation flow is found around the straightener, and several dead zones of ammonia mass fraction distributions are found at the corners of the catalyst layers. The mixing quality is improved significantly by incorporating a series of gate leaves. Predicted transient turbulence information by LES may be important for the optimization of the SCR reactor structure. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.