In the Selective Catalytic Reduction (SCR) system of a diesel engine, the decomposition of urea will produce the reducing agent NH3, and by-products which will form deposits. To reduce deposits, this work studied the relationship between liquid film temperature and reactants. Further, the mixers with different structures and the injectors with different injection parameters were designed and compared with different designs about the location and mass of deposits. The evaporation and pyrolysis process of urea aqueous solution in an SCR system were simulated by using computational fluid dynamics (CFD) coupled with a detailed urea decomposition mechanism. The results indicated that the by-products (biuret, CYA, and ammelide) would be produced at 300, 380, and 437 K, respectively. With increasing liquid film temperature, the mass fraction of NCO- and by-products also increased. The turbulence intensity varied when they were generated by the different mixer structures. In lower turbulence intensity of mixer1, with the decrease of injection angle and pressure, the mass and distribution regions of liquid film and deposits were obviously reduced. In higher turbulence intensity of mixer2, the urea droplets were entrained by strong turbulence. The decrease of the injection angle and pressure did not significantly reduce the liquid film and deposits. (c) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.