Coilform group is a group of short rod-shaped bacteria, which normally parasitizes animal intestines. There are also many Coliform group bacteria found in feces. Most of these bacteria wii not cause disease, but can serve as an index for water polluted by feces. Coliform group bacteria are no-spore bacilli that can decompose Lactose and produce gas, which are aerobic and facultative; it shows Gram Negative under the observation of a microscope. Coliform cannot directly reproduce in water, and the feces of warm-blooded animals generally contain this kind of bacteria, therefore, if we can detect large number of Coliform in water, it indicates water recently polluted by human or animal feces. Coliform and other pathogenic bacteria all come from warm-blooded animals and Coliform’s survival time is longer than pathogenic bacteria’s.Therefore if Coliform bacteria cannot be detected in water, the probability that the water contains other pathogenic bacteria is lower. Coliform group is a common biological index to estimate water quality.