Preparation method of black reactive dye

The invention relates to a preparation method of a black reactive dye. The structure of the dye is represented by a formula shown in the specification; and in the formula, Z is a SO2Y, Y is a vinyl group or a CH2CH2U group, and U for is a group which can leave under an alkaline condition; and a Z position can be a meta or para position, and M is hydrogen, an alkali metal or alkaline earth metal. Aminophenyl para or meta-(beta-hydroxyethyl sulfone sulfate) is diazotized to obtain a diazo salt, and acid coupling of the diazo salt and an H acid is carried out at 10-15DEG C until the diazo salt is completely reacted in order to obtain an acid conjugate; and alkaline coupling of the acid conjugate and diazotized aminophenyl para or meta-(beta-hydroxyethyl sulfone sulfate) is carried out, desalination is carried out, and then spray drying is carried out. The reactive dye has excellent dyeing performance and level dyeing property, and the maximum absorption wavelength lambda max of the dye is in a range of 605-610 nm and is longer than the wavelength of C.I. reactive black 5, so the black reactive dye has good compatibility with other component dyes, the buildup property is improved, fabrics dyed by using the black reactive dye have excellent blackening degree and dyeing levelness, and the color fixation rate is higher than that of the C.I. reactive black 5. The black reactive dye is used for dip dyeing and pad dyeing of fiber fabrics.