The invention relates to a blue reactive dye and a preparation method thereof. The structural formula (I) of the blue reactive dye is shown in the specification. The prepared blue reactive dye is improved in color fixing rate, alkali resistance and solarization fastness and improved in solubility, and has the obvious advantage that the alkali resistance of the blue reactive dye is capable of satisfying the strong-basicity requirement of cold pad batch. The blue reactive dye is applicable to dyeing of cotton, flax and wool fiber.