1. A flame detection method based on image processing comprising following steps: (a) capturing images of a predetermined area by an image capturing device; (b) detecting number of flame pixels of each image by determining if each pixel of each image satisfies the following conditions: R>R T ; R≧G>B; S≧((255−R)*S T /R T ); wherein R, G, B are red, green, blue gray levels of the pixel respectively, R T is a threshold of the red gray level, S is saturation of the pixel, and S T is saturation of the pixel when the red gray level of the pixel equals R T ; (c) generating a value according to the number of flame pixels of each image by a fuzzy system; and (d) comparing values generated from images captured within a predetermined time interval to generate a comparison result.