In order to clarify the role of HA (humic acid)-TiO2 adsorption in the photocatalytic process, a series of experiments were performed to investigate the adsorption mechanisms in the absence or presence of calcium according to the intramolecular interaction force theory. Based on the experimental results, the models of adsorption mechanism were designed, which were useful in explaining the phenomena that happened during adsorption processes. The adsorption of humic acid onto the TiO2 particles was desperately pH-dependent; however, calcium could increase the amount of adsorption, which was mainly attributed to the calcium ion bridging. The effects of calcium concentration and TiO2 dosage on the adsorption process are also discussed. In order to clarify the role of HA (humic acid) -TiO2 adsorption in the photocatalytic process, a series of experiments were investigated to the adsorption mechanisms in the absence or presence of calcium according to the intramolecular interaction force theory. Based on the experimental results, the models of adsorption mechanism were designed, which were useful in explaining the phenomena that happened during adsorption processes. The adsorption of humic acid onto the TiO2 particles was desperately pH-dependent; however, calcium could increase the amount of adsorption, which was mainly attributed to the calcium ion bridging. The effects of calcium concentration and TiO2 dosage on the adsorption process are also discussed.