1. A multi-class object classifying method, comprising: (a) receiving and storing a plurality of classes, a plurality of first training images and a plurality of second training images, and extracting a plurality of first characteristic images from the first training images; using the first training images to generate a plurality of classifiers corresponding to the classes through a linear mapping classifying method, wherein the first training images and the second training images respectively correspond to the classes; (b) extracting a plurality of second characteristic images from the second training images and selecting a classifier from the classifiers; (c) using the classifier and the second characteristic images to determine a plurality of parameter ranges corresponding to the classes and a threshold corresponding to the classifier; (d) when two of the parameter ranges overlap, recording the remaining parameter ranges except for the two parameter ranges; after selecting another classifier from the classifiers except for the classifier that has been selected, repeating the step (c) until the parameter ranges don't overlap with each other; (e) when the parameter ranges don't overlap with each other, recording the parameter ranges; (f) receiving and storing a pending image and extracting a pending characteristic image from the pending image; using a classifier and the pending image to determine a pending parameter; (g) when the pending parameter is larger than the threshold corresponding to the classifier, classifying the pending image into the class corresponding to the classifier; and (h) when the pending parameter lies in the parameter ranges, classifying the pending image into the classes corresponding to the parameter ranges.