To determine the processability of plastics for injection molding, an apparatus was established based on the capillary rheometer in this work. It consists of a modified rheometer, a simplified mold unit, and a data acquisition system. The mold unit includes the mold rack, sprue bush, smooth board, and cavity board. Both boards have the inclined planes. They are coupled together to form the combined wedge. The mold clamping is simply achieved through inserting the combined wedge into the wedge trough of the mold rack. With the data acquisition system the real-time measurement is performed during the injection-molding test. With this novel apparatus the processability of plastics for injection molding can be directly characterized on the rheometer instead of the injection-molding machine, and the amount of the samples for experiments on the apparatus would be fewer than that on the injection-molding machine. Moreover, the apparatus can thoroughly monitor the injection flow for subsequent investigations. Additionally, the products of the experiments provide the standard strips required for the mechanical properties tests of most experimental materials. By means of this apparatus, a conventional capillary rheometer can be modified as a convenient tool for the development of new materials.