Abstract: Humic acids from different fertilization were fractionated by alcohol precipitation technique, complexation characteristic of the fractions with Fe2+ were studied on the basis of studying the properties of the fractions The results showed that the condensation degree of the fractions decreased and the structure became simpler with the fractional number increase Complex ability of the third fraction was the highest among all the 7 fractions, which increased from fraction 1 to fraction 3, and decreased from fraction 3 to fraction 7 This revealed that it was not benefit for complex if the molecular structure of the fraction of humic acid was too simple or too complicated, existing an optimum molecular state among the fractions for complex The complex ability of A type fraction of humic acid was higher than that of P type, if the A type fraction was too complicated, the opposite was true The complex ability of Rp type fraction of humic acid was lower than that of A or P type significantly The difference on complex ability of humic acids with different fertilization was related to their fractional composition variation and their fraction's complex ability