This study explored the fiber fracture length, fiber orientation, and melt flow-ability variations of long glass fiber composite materials under different plasticization parameter s. Polypropylene was selected as the molding material, and glass fibers with a length of 25 mm were selected for composite reinforcement. Using an ultra-long-fiber composite injection molding machine (with a three-stage plunger mechanism), injection molding experiments were conducted at different screw speeds and under back-pressures to explore fiber fracture variations. Fiber fractures during plasticization were also observed to provide a reference for setting parameters. Moreover, flow-length specimen molds with different gate designs and specimen thicknesses were designed to facilitate observation of the effect of plasticizing properties on the flow-ability of long glass fiber composite materials. The experimental results revealed that fiber length decreased as screw speed increased. Under the parameter of low screw speed and without back-pressure, 60% of the fiber length was retained. Under the parameter of high screw speed and back-pressure, fibers fractured and the fiber length was shorter than 10 mm. In addition, during the filling process of fiber-containing plastic material, the thickness of the product and the gate design affected the filling process. The thin-wall of the mold-cavity product increased the flow resistance and affected the flow-ability and fiber distribution of long glass fiber composite materials.