A model of urea-SCR system was built to simulate low temperature performances. The results reveal that, in the range of 150~250 ℃,gas hourly space velocity(GHSV) has more impact on DeNOx than at a higher temperature; when GHSV is smaller than a value, DeNOx can hardly change; and below 200 ℃,the NH3 should be less than NOx, or it will lead to waste and pollution. The impact of the oxygen on DeNOx depends on the NO content, and this trend becomes weaker when the temperature increases. When the oxygen content reaches 5%, the DeNOx in different temperatures has stable values.