In this study, the adsorption behaviors of humic acid (HA) on ultrafiltration membrane surfaces and the HA adsorbed-layer structure were determined in the presence of Na+ and Mg2+, and the interaction forces of membrane HA and HA HA were measured. HA fouling experiments were performed to unravel the influence mechanisms of Na+ and Mg2+ on HA fouling. Our results show that the cations mostly affect HA fouling by controlling the electrostatic forces and hydration forces of membrane HA and HA HA. For Na+, HA fouling was mainly dominated by electrostatic forces. With increasing Na+ concentrations, the membrane HA and HA HA interaction forces increased because of the shielding effect, resulting in a higher deposition rate of HA onto the membrane surface and a more compact HA layer, accompanied by more serious membrane fouling. Mg2+ affected HA fouling in two stages. Lower Mg2+ concentrations acted similarly to Na+, with membrane fouling dominated by the electrostatic interaction forces. However, when the Mg2+ concentration exceeded a critical value, hydration repulsion forces caused a decrease in the deposition rate and the extent of HA on the membrane surface, and a less rigid HA layer was formed; membrane fouling was therefore mitigated.