The glass or carbon fibers have been verified that can enhance the mechanical properties of the polymeric composite injection molding parts due to their orientation distribution. However, the interaction between flow and fiber is still not fully understood yet, especially for the flow-fiber coupling effect. In this study, we have tried to investigate the flow-fiber coupling effect on fiber reinforced plastics (FRP) injection parts utilizing a more complicated geometry system with three ASTM D638 specimens. The study methods include both numerical simulation and experimental observation. Results showed that in the presence of flow-fiber coupling effect the melt flow front advancement presents some variation, specifically the “convex-flat-flat” pattern will change to “convex-flat-concave” pattern. Furthermore, through the fiber orientation distribution (FOD) study, the flow-fiber coupling effect is not significant at the near gate region (RG). It might result from the strong shear force to repress the appearance of the flow-fiber interaction. However, at the end of filling region (ER), the flow-fiber coupling effect tries to diminish the flow direction orientation tensor component A11 and enhance the cross-flow orientation tensor component A22 simultaneously. It results in the dominance at cross-flow direction at the ER. This orientation distribution behavior variation has been verified using micro-computerized tomography (micro-CT) scan and images analysis technology.