This paper proposes a numerical approach for predicting the injection-molding process of short-fiber-reinforced plastics using the Moving Particle Semi-implicit (MPS) particle-simulation method. Unlike conventional methods, this approach represents all of the fibers and the resin as particles and automatically analyzes the interaction between the fiber and the resin and the interaction between the fibers. This method can also follow the flow of a specific fiber. The injection molding of short-fiber-reinforced plastics was simulated assuming the thermoplastic resin to be an incompressible viscous fluid and the fibers to be rigid bodies. The numerical result demonstrated that the molding material was unidirectionally reinforced by short fibers since the fibers were rotated and aligned parallel to the flow direction due to the velocity gradient near the boundary. Moreover, the resin was predicted to accumulate at a corner. These results agreed well with previous studies, and the present approach was confirmed. Furthermore, we predicted the accumulation of fibers near a wall due to the velocity gradient, which could not be represented by conventional simulation methods.