The invention discloses a preparation method of reactive dye. A reactive dye preparation device applied to the method comprises a machine body fixed on a support, the machine body is internally provided with a first sliding groove, the first sliding groove with an upward opening extends leftwards and rightwards, a top seat extending upwardly is integrally arranged on the right side of the upper end of the machine body, and a second sliding groove with a downward opening is formed in the top seat and is communicated with the first sliding groove; a cylinder is fixedly mounted on the upper end face of the top seat, a feeding groove communicated with the cylinder and the first sliding groove are formed in the top seat; a reaction cylinder capable of sliding leftwards and rightwards is mountedin the first sliding groove; a cylinder cavity with an upward opening is formed in the reaction cylinder; first guide grooves extending leftwards and rightwards are symmetrically formed in the frontend wall of the first sliding groove, and first guide blocks fixedly connected with the reaction cylinder are slidably arranged in the first guide grooves; first threaded rods are fittingly mounted inthe first guide blocks.