The invention provides a cellulosic fiber reactive dye dyeing method. The method comprises two processes of preprocessing and dyeing a cellulosic fiber yarn. A strongly polar aprotic solvent is used to preprocess the cellulosic fiber yarn, the processing temperature is 60-110 DEG C, and the processing time is 0.5-6 hours; and the preprocessed cellulosic fiber yarn is dyed in a ternary mixed solvent of the strongly polar solvent, a weakly polar solvent and water, the volume ratio of the strongly polar solvent to the weakly polar solvent to the water is 1:(10-30):(0.1-3), the mass ratio of the fiber yarn to the ternary mixed solvent is 1:(8-20), the dye has an excellent solubility in the ternary mixed solvent, the use level of the dye is 0.5-8%(o.w.f.), the use level of an inorganic base is5-25 g/L, the dyeing temperature is 40-100 DEG C, the heat preservation time for the dyeing is 1-3 hours, and the dyed fiber yarn is obtained after centrifuging, washing and drying. The dyeing methodis salt-free, uses less water, and is a reactive dye solvent dyeing method being high in dye uptake and color fixing rate.