Abrasive waterjet (AWJ) fracturing has become an accepted horizontal multistage stimulation technique due to its flexibility and high efficiency of extensive fracture placement. The downhole tool failure of AWJ fracturing becomes an issue in the massive hydraulic fracturing because of high velocity and proppant erosion. This paper proposed a 3D computational fluid dynamics (CFD)-based erosion model by considering high-velocity waterjet impact, proppant shear erosion, and specific inner structure of hydra-jet tool body. The discrete phase approach was used to track the proppant transport and its concentration distribution. Field observation provides strong evidence of erosion patterns and mechanisms obtained from CFD simulation. The results show that the erosion rate has a space dependence in the inner wall of the tool body. The severe erosion areas are primarily located at the entries of the nozzle. Evident erosion patterns are found including a ‘Rabbit’s ear’ erosion at the upper-layer nozzles and a half bottom loop erosion at the lower-layer nozzles. Erosion mechanisms attribute to high flow velocity at the entry of nozzles and the inertia force of proppant. Sensitivity analysis demonstrates that the pumping rate is a primary factor contributing to erosion intensity.