The water treatment agent composited by the sodium alginate encapsulated chitosan gel beads and the nano calcium peroxide gel beads coated with sodium alginate/polyethylene glycol 4000 were prepared to investigate its performance and mechanisms in the removal of phosphorus and organic matters in water. The physicochemical properties and morphology of the agent were characterized by X-ray diffraction, scanning electron microscope and elemental energy dispersive spectrometer. This composited water treatment agent can effectively remove single phosphorus or organic matters in water, and also has high removal efficiency for the complex phosphorus and methyl orange pollutants. The water treatment agent has higher adsorptive ability for phosphorus at high concentrationa real river. The slow release of oxygen can maintain the mass concentration of dissolved oxygen above 4. 5 mg/L, therefore providing better aerobic condition for microorganisms in muddy-water interface and elevating the ability of self-purification of water.