Abstract: The complex stability characteristics of three humus fractions with Hg were studied.The results show that Hg can either be strongly bound or weakly bound with the humus.Fulvic acid bound Hg is mainly in a weakly bound form,the grey humic and brown humic acid bound Hg are dominantly in a strongly bound form.Of the three humus fractions,fulvic acid has the largest complex capacity for Hg,but due to its lowest complex stability constant,fulvic acid bound Hg will have the highest environmental activity .The complex stability constant of grey humic acid for Hg is the highest so the Hg environmental activity is the lowest. But due to its limited complex capacity,the complex stability constant of grey humic ac id with Hg will greatly drop with the increase of the mass ratio of Hg/grey humic acid.The complex characte ristics of brown humic acid lies the medium between those of fulvic acid and grey humic acid.