Novel carboxyl-functionalized magnetic Fe3O4 nanoparticles coated by poly 2,3-diaminophenol (CMNP@PDAPs) were prepared by a facile method. The core-shell CMNP@PDAPs can efficiently remove Cr(VI) and As(V)/As(III) from solution, especially in the bi-solute systems. The reduction of Cr(VI) to less toxic Cr(III) by amine are found to be the driving force for Cr(VI) adsorption. For the adsorption of As(III), the inner-sphere surface complexation mechanism dominate the adsorption process, while the electrostatic outer-sphere dominates the As(V) adsorption on CMNP@PDAPs. Interestingly in the bi-solute systems, the As(III)/As(V) removal was promoted by Cr(VI) while the Cr(VI) removal was decreased. X-ray photoelectron spectroscopy (XPS) analysis indicates the reduction of Cr(VI), and the interactions between the chelated Cr(III) and As(V)/As(III) results in the enhanced As(V)/As(III) adsorption. Based on theoretical density functional theory (DFT) calculations, the energy gap of HOMO-LUMO is in the order of Delta E-gap(H3AsO3)>Delta E-gap(H2AsO4-)>Delta E-gap(HCrO4-), further confirms the Cr(VI) is reduced to Cr(III). The transition states for the conversion of Cr(VI) to Cr(III) and the optimized geometries of the reaction intermediates were also highlighted. The CMNP@PDAPs show great potential in simultaneous removal of Cr(VI) and As(V)/As(III) from seriously polluted water with muti-pollutants.