Abstract: Analyzing the problems found in the conventional median filters applied to the image pre-processing of injection-molding for plastic products, a novel multi-stage median filter is proposed to suit well the images plastic products. By the training of a SVM-based sorter, the type of pixels in image can be distinguished from each other so as to enable the new filter to choose properly a filtering algorithm in accordance to the type of pixels. In this way the SVM-based multi-stage median filter offers an excellent capability for edge detail protection with noise suppressed efficiently, thus satisfying the requirements of the image pre-processing of plastic products. Simulation tests have verified the effectiveness of the new filter.