The invention discloses a preparation method of high substitution soluble phosphate starch, which comprises dissolving potassium carbonate into water, mixing and adding potassium dihydrogen phosphate, when all bubbles disappear, adding urea and dissolving, preparing esterifying liquid, adding the esterifying liquid into starch to be mixed uniformly, drying at 45-55DEG C, increasing temperature to135-150DEG C, reacting for 3-5h at constant temperature, to obtain the high substitution soluble phosphate starch whose substitution is higher than 0.08. The prepared high substitution soluble phosphate starch is soluble in cold water, with high substitution and better water adsorption, which contains three nutrient elements as nitrogen, phosphorous and potassium needed by the growth of crops, therefore, the product can be used in agriculture and forestry.