To explore the structural characteristics of different molecular weight humic acid fractions, lignite powders was used to extract humic acid for ultrafiltration classification, and four humic acid fractions with different molecular weights were obtained. It is found that phytic acid with molecular weight greater than 50 kDa and less than 1 kDa account for 49.66% and 47.07% of total phyphyic acid, respectively, and the middle two components are very little. The physicochemical properties and function of phytic acid are mainly determined by phyphyic acid below 1 kDa and above 50 kDa. Humic acid below 1 kDa have the highest contents of total acid group, carboxyl group and phenolic hydroxyl group. Compared with carboxyl group, phenolic hydroxyl group accounts for a higher proportion of total acid group. The degrees of aromatization and condensation of humic acids are increased with the increase of molecular weight, while the content of oxygen-containing functional groups and the value of E₄/E₆ are decreased with the increase of molecular weight. The pot experiment was used to study the effects of different molecular weight humic acids on Cd availability in soil. The adsorption amount of Cd is decreased with the molecular weight of humtic acid, reaching adsorption equilibrium at 500 mg/L. And the desorption of soil Cd also decreased with the increase of the molecular weight of humtic acid. Small molecular weight humic acids rich in acidic functional groups, thus have stronger adsorption and complexation abilities of soil heavy metals, thus can reduce soil Cd adsorption capacity, improve its mobility and bioavailability; while large molecular humic acids mainly acts as adsorbent and immobilization, which can reduce the mobility and plant availability of heavy metals.