Laboratory incubation experiments in constant temperature were implemented to study influence of humic-acid and fulvic-acid on fixation and release of potassium in cultivated brown soil.The results indicate that application of humic acids accelerated potassium release from the soil, showing a tendency of the rate of K release increasing with the rate of humic acid addition.After incubating at 25℃ for 21days, K release increased by 6.25% in the soil treated with 40 g kg^-1 fulvic-acid, but only by 2.26% in the soil treated with humic-acid.Application of humic acids may reduce the rate of the soil fixing extraneous K.After application of 400 mg kg^-1 K, the soils treated with humic acids differed significant from the soil in the control in Kfixation.But the effect of humic acids reducing K fixation rate was affected by how K was applied.In the treatments where humic acids-treated soils were incubated in constant temperature for 7days before application of potassium, the K fixation rates were lower than that in the treatment of applying potassium and humic acids at the same time.The effect of humic acids on fixation and release of potassium in soil was accomplished by dissolving K-bearing minerals or blocking interlayers and adsorbing K.