NOVELTY - Preparing an organic seaweed fertilizer comprises taking fresh or dried macroalgae and cultivating, adding water, sodium and heavy metal activated sludge to the macroalgae, uniformly mixing, adding brokenstraw, bran and animal urine to the mixture, crushing the mixture, spraying a corrosion accelerating agent on the mixture, composting the mixture at less than 65 degrees C, fermenting the mixture for 1-5 days, spraying effective microorganism on the mixture, adding a seaweed residue to the mixture, fermenting the mixture for 15-25 days, granulating and cooling the mixture, and drying.    USE - The method is useful for preparing an organic seaweed fertilizer (claimed), which is useful for promoting growth of plant.    ADVANTAGE - The method prepares the organic seaweed fertilizer with improved storage stability, increased soil fertility and water storage capacity and reduced environmental pollution and cost.    DETAILED DESCRIPTION - Preparing an organic seaweed fertilizer comprises taking fresh or dried macroalgae, cultivating the macroalgae and then processing, adding 55-65 wt.% of water, 25-30 wt.% of sodium and 55-65 wt.% of heavy metal activated sludge to the processed macroalgae, uniformly mixing, measuring organic matter and nitrogen contents, adding brokenstraw, bran and a mixed animal urine to the mixture, crushing the mixture, spraying a corrosion accelerating agent on the crushed mixture, uniformly mixing, measuring the temperature using a thermometer at 50-55 degrees C, composting the mixture at less than 65 degrees C, sealing the composted mixture, fermenting the sealed mixture for 1-5 days under an anaerobic environment, further composting the fermented mixture for 10-15 days, spraying effective microorganism on the composted mixture under an anaerobic environment, measuring water content, adding a seaweed residue to the mixture, fermenting the mixture having a moisture content of 55-65% for 15-25 days, sieving the fermented mixture, crushing the sieved mixture, composting the crushed mixture, drying the composted mixture and then ventilating, controlling the water content of 10-15%, granulating the mixture, sieving the granulated mixture, naturally cooling the sieved mixture at room temperature, and drying. An INDEPENDENT CLAIM is included for the organic seaweed fertilizer prepared by the above method.