The invention discloses a preparation method of large-granule magnesium ethylate. The method comprises the following steps: adding 2-4 parts of iodine and 50-60 parts of ethanol into a three-necked flask, raising the temperature of the flask to 44-46 DEG C at a rotating speed of 300 r/min, adding 20-30 parts of magnesium powder and 35-45 parts of ethanol in six times, allowing the interval time tobe 50-60 min, carrying out a reaction for 2-4 h after the addition operation is completed, washing the obtained reaction product with hexane at normal temperature 4-6 times after the reaction ends, and carrying out vacuum drying at 45-55 DEG C. The method has the advantages of simplicity, fastness, easiness in operation, successful preparation of the large-granule magnesium ethylate with a good quality and a high bulk density, and provision of necessary conditions for a catalyst needed by the production of high-density polyethylene.