Several models involving spray atomization, wall impingement, evaporation and urea pyrolysis were built through a commercial code to study the influence of parameters of Urea Selective Catalytic Reduction (Urea –SCR) injection system on critical performances at low exhaust temperatures. The spray development,NH3 concentration distribution and wall film thickness have been studied. Results suggest that the wall impingement of the Urea Water Solution (UWS) is more at the temperature of 180°C than at 430°C. Lower temperatures tend to hinder evaporation and pyrolysis. NH3 concentration is uneven at low temperatures and the fuel film generated on the wall is also incomplete. Low temperatures could help injection system with a tilting nozzle raise its injection pressure and accelerate the evaporation and pyrolysis of the droplets. The distributionof spray impingement, however, is narrower and the fuel film thickness on the wall is increased. Smaller injection angle made it easier to expel gas stream and the droplets would then follow the direction of the gas stream. The droplets distribution is therefore more uniform and the spray wall-impingement reduced.