Based on Taguchi experimental design method, the L25(56) orthogonal matrix has been adopted for an experimental design, a 3-D numerical simulation of the injection molding process of short-glass fiber reinforced PA66 composites has been carried out, and a research on the influence of different process parameters of the injection molding and interaction coefficients of glass fiber on the orientation tensor of injection molded fibers has been conducted by making use of Moldflow software. The experimental result shows that the fiber interaction coefficient (Ci) has a significant effect on fiber orientation, with a contribution rate as high as 99.999 994 57%. Within the range of experimental parameters, the average fiber orientation tensor decreases monotonically as the fiber interaction coefficient (Ci) increases; while the fiber orientation tensor is under little influence of five injection molding processing parameters, namely, the mold temperature, the melt temperature, the injection time, the packing pressure and packing pressure time. Meanwhile, with fiber interaction coefficients the independent design variables, and by making use of Newton linear difference, a 4 order regression equation of the fiber orientation tensor of injection molded parts has thus been established. This equation can provide references for the prediction of the fiber orientation at the principal stress direction of injection-molded parts and the determination and evaluation of the nature of the intensity enhancement effect.