In this work, we found that ZnCl solution can not only be used as a plasticizer for starch but also provide a mechanical reinforcement effect to the resultant starch-based materials. By a one-step compression molding process, well-plasticized starch-based films could be obtained at 120 °C with a 15 wt.% ZnCl solution. Both the tensile strength and elongation at break of the films increased with a rise in ZnCl concentration, which demonstrates a mechanical reinforcement. This reinforcement could be mainly ascribed to the in-situ formed starch-zinc complexes and the enhanced starch molecular interactions. Moreover, if the processing method was changed into firstly mixing followed by compression molding, the tensile strength increased by more than three folds at no cost of the elongation at break. Regarding this, we propose that shear could further enhance the molecular interactions within the material. However, if the ZnCl concentration was too high, the mechanical properties were then reduced irrespective of the processing protocol, which could be due to the weakened molecular interactions by ZnCl. Thus, we have demonstrated a new, simple method for preparing starch-based composite materials with enhanced mechanical properties, which could be potentially applied to many fields such as packaging, coating and biomedical materials.