The invention discloses a reduction system for vat dye or sulfur dyestuff dyeing and a reduction method. The reduction system is aqueous solution of thiourea dioxide and weak base, that is, the thiourea dioxide and weak base serve as solute, water serves as solvent, and the weak base is at least one selected from sodium carbonate, trisodium phosphate or sodium silicate; the concentration of the thiourea dioxide is 0.05 mol / L-0.2 mol / L, and the concentration of the weak base is 0.01 mol / L-0.10 mol / L. The reduction method comprises the following steps: adding the dye in the reduction system to prepare dye leuco body solution, heating, dip dyeing textile, and performing after treatment. According to the invention, a selected weak base system is adopted to replace strong base, so that the stability of a reduction leuco body is improved, the dyeing effect is enhanced, the dye-uptake is higher than that of the traditional sodium hydrosulfite-caustic soda method, the color fastness and the color uniformity are better, and the use is safe.