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, extracting a plurality of first characteristic images from the first training images, and 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, and 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, and 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.