Selective catalytic reduction (SCR) has become one of the primary technologies to reduce internal combustion engine (ICE) emission. The installation angle of urea injector plays an important role during the SCR process. The urea injector is often vertically mounted to the exhaust pipe for on road heavy duty truck because of its good performance and general packaging convenience, and this type of installation has been the focus of previous research. However, due to certain packaging constraints or responsiveness considerations, the injector is installed with an inclined acute angle to the exhaust pipe under some circumstance. To evaluate the underlying benefits and risks of this type of installation angle, a computational fluid dynamic (CFD) model based on the Renolds averaged Navier-Stokes (RANS) solver from AVL Fire is used to simulate the injection process of urea for an acute-angled 3-hole injector, through which, the urea spray's formation and motion, wallfilm accumulation and NH3 distribution uniformity characteristics are studied. Result shows that with an inclined injector, the sprays are blown off from the centerline of the pipe in different operating conditions that lead to an early spray impingement with pipe wall and a postponed evaporation time. An inverse gas flow region is also found near the mounting flange of the injector that entrains the wallfilm upward, accumulating thick wallfilm with high probability of crystallization that has been confirmed by previous vehicle malfunction report. In part II of this research, orthogonalization method is used to find out the optimized combination of inclination angle, circumferential angle and injector reach within the limited installation space of the vehicle. Three operating conditions are considered with different weight according to their working time on road. The orthogonalization experiment schemes are calculated by the CFD model and the optimized combination of installation parameters is found. Result shows that it can improve the injection process with both urea wallfilm area and response time reduced.