On the basis of studying the properties of humic acid in the soil with application of corn straw in different decaying time, the alcohol precipitation technique was used for classifying soil humic acid into 7 fractions and the fraction composition of humic acids and their properties were researched. The results showed that the type of humic acid changed from type A to B and then to A during the whole decaying periods of corn straw. Humic acid changed from complex to simple and then to complex again. The fr...