Based on the physical and chemical properties of silk and flax fibers, different reactive dyes are selected in union-color of one bath dyeing. For evaluating the dye-ability of the blended fabrics, reactive dyes containing bi-functional reactive groups are found. On basis of a number of experiments, influences of fixing temperature, fixing time, sodium sulfate consumption and sodium carbonate dosage are discussed. Through orthogonal experiment，the optimum conditions to silk/flax blended fabrics with Reactive Yellow B-4GLN are defined. Results of performance test indicate that Reactive Yellow B-4GLN has good colorfastness and levels and the union-color property of silk/flax fabrics is well.