For better understanding of thin-wall injection molding processes, a visualization injection mold and high speed camera were used to observe the dynamic filling behavior of melt in cavities with different thicknesses during the injection molding process. The impact of the scale effect on the melt-filling rate and filling imbalance in the multi-cavity were also studied. Results indicated that the filling behavior of the melt during the multi-cavity molding process was related to the cavity thickness. The experiment results can be used to describe the filling imbalance phenomenon during the injection molding process and to provide experimental evidence for precise injection molding technology at the micro scale.