Polystyrene microplastics (PSMPs) are detrimental in aqueous environments. This study found that humus, mainly comprising humic acid (HA) and fulvic acid (FA), can facilitate the adsorption of As(iii) by PSMPs. The phenolic hydroxyl groups provided by HA contribute to the transport of As(iii). HA interacts with the PSMPs to form a π complex, providing more sites on the microplastics for As(iii) adsorption, while reducing the time required to reach adsorption equilibrium. Increased temperatures in aqueous environments destroy the hydrogen bonds contributing to the adsorption process, thus causing desorption. Increases in pH and ionic strength reduce the adsorption of As(iii) by increasing charge repulsion and microplastic agglomeration, and the co-existing NO3- and PO43- ions inhibit the removal of As(iii) in the solution. Our combined results indicate that the migration of PSMPs after As(iii) adsorption in the presence of HA and FA requires further research attention.