Effects of injection molding parameters on the residual fiber length and the mechanical properties of LFRT-PP composites, which were prepared by self-designed impregnating molding, were investigated by orthogonal experimental design method. The results showed: the length of residual glass fiber increased with the length of LFRT pellets and decreased with screw speed. The former had more remarkable impact. The mechanical properties of composites were greatly affected by the temperature of barrel. The lower temperature was benefit for obtaining the higher properties. The length of residual glass fiber was not directly corresponding with the mechanical properties. To obtain the highest mechanical properties of composites, the optimal parameter levels were: the length of pellets 10mm, the temperature of barrel 220°C, the screw speed 75 rpm and the injection pressure 80 MPa.