The invention provides a liquid red direct dye and a preparation method thereof, and the method comprises the following steps: S1, dispersing 2-amino-5-naphthol-7-
sulfonic acid and bis (trichloromethyl)
carbonate in water, adjusting the pH value of a
reaction system by using a first
organic base, and reacting to obtain a scarlet acid
aqueous solution; s2, dispersing a sulfo
aromatic amine compound and
nitrite in water, adjusting the pH value of a
reaction system by using inorganic acid, reacting to obtain sulfo
aromatic amine diazonium salt precipitate, and performing
solid-liquid separation to obtain sulfo
aromatic amine diazonium salt; and S3, adding the sulfonic aromatic amine diazonium salt in the step S2 into the scarlet acid
aqueous solution in the step S1, adjusting the pH value of the
reaction system by using second organic alkali, and reacting to obtain the liquid red direct dye. Through the preparation method, the red direct dye which can stably exist in a liquid form and is suitable for direct use can be obtained.