NOVELTY - Collecting algae by magnetic flocculation nano-particle, comprises firstly adjusting pH value of micro algae culture solution to be greater than or equal to 6, adding magnetic flocculation nanoparticles into the micro algae culture solution, mixing and stirring to obtain polymer of micro algae cells and the magnetic flocculation nanoparticles, carrying out magnetic adsorption on the polymer of the micro algae cells and the magnetic flocculation nanoparticles by a magnetic field generator, and finally separating.    USE - The method is useful for collecting large scale algae, which is useful for treating sewage (all claimed).    ADVANTAGE - The method rapidly collects algae without damage and within 4 minutes in a simple, environmental friendly and cost-effective manner.    DETAILED DESCRIPTION - Collecting algae by magnetic flocculation nanoparticle, comprises: (a) firstly adjusting pH value of micro algae culture solution to be greater than or equal to 6, adding the magnetic flocculation nanoparticles into the micro algae culture solution, mixing and stirring to obtain polymer of micro algae cells and the magnetic flocculation nanoparticles; and (b) carrying out magnetic adsorption on the polymer of the micro algae cells and the magnetic flocculation nanoparticles by a magnetic field generator, and finally separating to obtain the polymer of the micro algae cells and the magnetic flocculation nanoparticles; where the magnetic flocculation nanoparticle is obtained by ultrasonically dispersing ferroferric oxide nanoparticles into chitosan acetic acid solution, adjusting pH value of the solution to be 4, dropping sodium tripolyphosphate solution to adjust the pH value to be 7-7.5, and finishing dropping to continuously react, where the mass ratio of ferroferric oxide nanoparticle to chitosan to sodium tripolyphosphate is 5:0.6:0.06. An INDEPENDENT CLAIM is included for treating sewage comprising adding micro algae mother liquor into sewage treatment pool, cultivating for 3-4 days in outdoor using standard water body nitrogen, and discharging the sewage after processing.