The invention especially relates to a preparation method for high-purity amino trimethylene phosphonic acid, which belongs to the technical field of chemical engineering. The method comprises the following steps: step 1, pumping phosphorous acid and concentrated hydrochloric acid into a reaction vessel and putting ammonium chloride into the reaction vessel; step 2, starting a steam jacket for heating to a temperature of 100 DEG C, strictly controlling temperature in the process of a reaction to be more than 100 DEG C and slowing adding 35 to 37% by mass of formaldehyde drop by drop; step 3, controlling a temperature to be more than 100 DEG C and maintaining the temperature for 1 to 3 h; step 4, concentrating a material until water content is 15 to 25%; and step 5, carrying out centrifugation so as to obtain high-purity amino trimethylene phosphonic acid; wherein a weight ratio of phosphorous acid to concentrated hydrochloric acid to ammonium chloride to formaldehyde is 2520: 850: 625: 3015. According to the invention, scientific process is employed for product purification; and amino trimethylene phosphonic acid in the prepared high-purity amino trimethylene phosphonic acid is more than 95%.