Abstract: An injection molding system has a plurality of movable sub-clamping units each having a mold therein and a main clamping unit for clamping the mold in the sub-clamping unit. Each of the sub-clamping units alternately enters the main clamping unit for only a filling process to receive the melted resin material from an injection unit. Each sub-clamping unit clamps the mold by its own clamping mechanism and moves to the main clamping unit where the mold is further clamped by a higher clamping pressure by the main clamping unit during the filling process. Each sub-clamping unit is retracted from the main clamping unit immediately after the filling process and stayed outside of the main clamping unit through the holding process and the cooling process, thereby dramatically improving the molding production efficiency.