Due to the lack of experimental methods, there is little direct and reliable experimental basis for the theoretical study of melt flow characteristics in the process of co-injection molding. In this study, filling processes of PP melt of co-injection molding were directly observed by self-developed visual experimental device. In order to analyze the filling characteristics, results of the co-injection molding were compared with that of injection molding process in metal cavity with the same structure and molding parameters. The results show that the flow pattern of melt filling in co-injection molding can be different from that of conventional injection molding. The melt temperature is the main factor that affects the characteristics of co-injection molding, and the different temperature of the melt will show different forming characteristics at different injection rates.