Abstract: Fractal theory studies the widespread disordered (ruleless) system with self-similarity in the nature and social activities. Within the earth system, the spatial distribution of many geo-scientific phenomena displays fractal structure. As a kind of geological record, alteration is clearly not homogeneous. Meanwhile alteration also can be regarded as a kind of anomaly, and fractal dimension is usually linked with anomaly. The summation method (namely pixel grayscale and area mode) based on fractal theory is an effective way to extract and grade alteration anomaly from the background. This method can make up the insufficiency of conventional threshold method and identification of catastrophe point using grayscale histogram.