Self-reinforced polymer composites (SRCs), which are fabricated by combining the same type of polymer with different properties into one body, have high specific strength, no interfacial heterogeneity, and ease of recycling. To better understand the relationship between the molding process and mechanical properties of SRCs, the co-injection molding process was used in this study to process SRCs samples. Further, a self-developed visualized experimental device was used to observe the flow of the polymer melt directly during co-injection molding. From the visualization results, it was found that the tensile properties of SRCs are positively correlated with the absolute value of the slopes of the velocity change at lower melt temperature. When the melt temperature increases to a certain degree, the tensile properties of SRCs are much lower than that of lower melt temperature. The shear rate inside the melt flow and the shear rate between the melt and the matrix wall play a major role in the self-reinforcing characteristics of the SRC.