Weight-loss testing and computational fluid dynamic simulation were combined to investigate the essential roles of fluid mechanics and the sand impact in erosion-corrosion (E-C) of an X65 pipe steel in oil sands slurry. Results demonstrated that the steel E-C is resulted from the synergistic effect of the hydrodynamic shear stress and mechanical impact stress exerted on the steel surface. At low impact angles, such as 30°, the effect of shear stress is dominant. The particle impact stress becomes dominant when the impact angle increases to a high value, such as 90°. It is demonstrated that the maximum of E-C of the steel occurs at approximately 45° of the slurry impingement.