An eutrophication water body in-situ electromagnetic coupling flocculation precipitation device comprises a storage battery, a solar cell panel and a buoy connected with the solar cell panel. The solar cell panel is connected with the storage battery, a sewage pump, a sludge pump, the storage battery, an air compressor and a magnetic seed feeder are arranged on the upper portion of the buoy, the sewage pump, the sludge pump, the air compressor and the magnetic seed feeder are connected with the storage battery, a flocculation reaction zone is arranged in the middle of the buoy, a sludge settling zone is located below the flocculation reaction zone, a precipitation separation zone is located at the two sides of the sludge settling zone, an eutrophication water body outside the buoy is pumped into the flocculation reaction zone through the sewage pump, the flocculation reaction zone is provided with multiple pairs of anodes and cathodes which are used for electric flocculation, a perforated pipe sparger which is connected with the air compressor is installed on the lower portion of the flocculation reaction zone, the magnetic seed feeder is used for feeding magnetic seeds into the flocculation reaction zone, and powerful magnets are installed on the two side surfaces of the sludge settling zone. According to the eutrophication water body in-situ electromagnetic coupling flocculation precipitation device, the running cost is low, the maintenance is simple, pharmaceutical pollution and solid waste are not produced, the magnetic seeds are easy to recycle, and concentrated sediment can be used as fertilizer directly.