The invention relates to a preparation method of 2'-[bis (phenylmethyl) amino]-6'-(diethylamino)-spiro [
isobenzofuran-1 (3H), 9'-[9H] xanthen]-3-one. The preparation method comprises the steps of conducting
condensation reaction in fuming
sulfuric acid by using 2-(hydroxyl-4-diethylamino-2-hydroxybenzoyl)
benzoic acid and N, N-diphenylmethyl-p-anisidine, then adding water and methylbenzene, conducting extraction and layering, removing an
acid water layer, adding alkali, conducting cyclization reaction, conducting heating and
distillation to remove methylbenzene, finally adding
methanol for refining and conducting
vacuum drying to obtain GREEN refined products. The preparation method has the advantages that since catalysts are added, the reaction yield of 2-(hydroxyl-4-diethylamino-2-hydroxybenzoyl)
benzoic acid is improved to 98 percent; since the fuming
sulfuric acid is used to replace concentrated
sulfuric acid, the amount of generated
sulfonate is reduced; and since
alkalinity pH (potential of
hydrogen) is controlled to be less than 12, the appearance characteristic and the yield of the products are improved and the product yield is improved to more than 82 percent.