Adsorbed humic acid (HA) on surfaces of nanoparticles (NPs) will affect their transport, transfer, and fate in the aquatic environment, especially in the presence of surfactants, and thereby potentially alter exposures and bioavailable fractions of NPs and surfactants. This study investigated adsorption of HA on Fe3O4 NPs in the presence or absence of surfactant. Surfactant established a bridge connecting HA and Fe3O4 NPs, and significantly changed adsorption behavior of HA on NPs. Adsorption of HA in the absence of surfactant was 120.3 mg/g, but 350.0 mg/g and 146.5 mg/g in the present of CTAB (hexadecyl trimethylammonium bromide) and SDS (sodium dodecyl sulfate), respectively. Surfactants can form different stages (hemimicelles, mixed hemimicelles and admicelles) on Fe3O4 NPs by electrostatic and hydrophobic interactions, adsorption of HA was different for each of those stages. Adsorption of HA on surface of Fe3O4 NPs/CTAB was codetermined by hydrophobic, electrostatic interactions and ligand exchange. The presence of CTAB or SDS changed mechanisms for adsorption and effects of functional groups. Results of Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) indicated that carbohydrate carbon was important in adsorption of HA on Fe3O4 NPs in the presence of surfactants.