A numerical model of spray and its interaction with the gas exhaust flow and static mixer are developed to investigate the effects of the mixer on the selective catalytic reduction (SCR). The residence time of spray droplets in straight pipes with and without mixers and the interaction between spray droplets and the blades of mixers are investigated. The numerical results show that due to the improvement of evaporation rates of sprays and the ability to break droplets into smaller ones, static mixers can help SCR systems achieve better urea conversion efficiency. Furthermore, the using of mixers significantly improves the uniformity of the reducing agent in a pipe.