The invention discloses a preparation method of
indigo carmine, which comprises the following steps: (1) adding
indigo into chloralkane, stirring and controlling the temperature, and dropwise adding
chlorosulfonic acid for sulfonation reaction; after the reaction is finished, controlling the temperature, adding deionized water, stirring, standing for layering, removing a lower-layer organic phase, adding liquid caustic soda into an upper-layer water phase under the
temperature control, separating out a
slurry solid until the pH value of a mixture of the
slurry solid and the solution is greater than or equal to 13, cooling to be less than or equal to 10 DEG C, and filtering; washing the
filter cake with cold deionized water and
methanol in sequence to obtain an
indigo carmine wet crude product; (2) adding the crude product obtained in the step (1) into deionized water, heating to 90 DEG C or above, completely dissolving the crude product, adding
activated carbon, stirring, filtering while hot, carrying out reduced pressure
distillation on filtrate, cooling to 10 DEG C or below, and filtering; and washing the
filter cake with cold deionized water and
acetone in sequence, and carrying out
vacuum drying to obtain an
indigo carmine product. The method has the advantages of simplicity in operation, high product yield, low cost and small amount of
wastewater and waste
solvent.