Disclosed are reactive dyes and preparation methods thereof. The reactive dye may be prepared using heterocyclic primary amine as a diazo component, 4,4′-diamino-2,2′-stilbenedisulfonic acid, s-triazine, or ethylenediamine as a bridging group, and a dimonochlortriazine group as an active group. The color-changing compound is covalently bonded into the fiber chemical structure by nucleophilic substitution reaction between monochlorotriazine and the hydroxyl group in a textile structure, and the size of the conjugated system and the range of electron delocalization of the dye are changed by a reversible isomerization reaction of the hydroxyl group adjacent to the diazo group and the diazo group under different pH regulations. Moreover, the dye has a double color changing structure, which improves the capability of the dye combined with —H/—OH. The pH value range that can cause color changes in the dye is effectively expanded to include weak alkali, weak acid or even neutral conditions.