An image classifying method includes the following operations: reducing an order of a color of a first image to generate a first order reduction image, wherein the first order reduction image includes several first image blocks; obtaining a second order reduction image from a database, wherein the second order reduction image includes several second image blocks; calculating several block color difference values between the first order reduction image and the second order reduction image respectively according to differences between a color value of each of the first image blocks and a color value of each of the second image blocks; and determining whether or not the first image belongs to a same category as the second order reduction image according to the block color difference values between the first order reduction image and the second order reduction image.