To meet recent stringent emission regulation, the Urea-SCR system should be installed on diesel vehicles.The Urea-SCR system consists of the urea water solution (UWS) injector and mixing chamber and the SCR catalyst. Injected UWS inevitably collides with the wall of the exhaust pipe or the mixer, and the most of the UWS particles rebound from solid surface after collision, but some of the UWS particles loss all their momentum and become wall-wetting state which increase the risk of urea solidification.Therefore, it is necessary to develop a mixing method that improves NH3 concentration uniformity and conversion efficiency and reduces wall-wetting in front of the SCR catalyst. One way to improve mixing performance is make injected UWS to collide with the wall of the mixer or the exhaust pipe in order to facilitate the dissociation of the UWS droplet, other way is using the mixer which forms the swirl flow to increase the relative speed difference between the exhaust gas and the UWS droplets. In this paper, 3D-CFD are performed by using commercial 3D code, STAR-CCM+ v12.02, and the behavior of the UWS droplets are closely analyzed by different mixing method and their intensity of effect. As a result, the behavior of the UWS droplets in the exhaust pipe is numerically calculated and the mixing method is proposed to improve NH3 concentration uniformity and conversion efficiency while reducing wall-wetting