Preparation method of alpha-naphtholphthalein

The invention discloses a preparation method of alpha-naphtholphthalein. The preparation method comprises the following steps of 1, a crushing process comprising pre-crushing phthalic anhydride and 1-naphthol into powder, 2, a reaction process, 3, a separation purification process comprising dissolving the viscous reactant by a solvent a, carrying out filtration, neutralizing the filtrate by an alkali solution, carrying out filtration, concentrating the filtrate to obtain a viscous liquid, and adding a solvent b into the viscous liquid, heating to boiling, carrying out cooling , carrying out filtration, adding deionized water into the filter cake, heating to boiling, carrying out filtration and washing, dissolving the filter cake by a solvent c, heating to boiling, carrying out strong resolving by deionized water, and carrying filtration and drying to obtain alpha-naphtholphthalein powder. The preparation method has simple processes and a high product yield, realizes high product purity and can be industrialized easily.