The invention discloses a treatment method for one bath of low-temperature bleaching and reactive dyeing of pure-cotton knitted fabrics, and belongs to the technical field of textile printing and dyeing. The treatment method provided by the invention adopts the following processes: (1) performing low-temperature activation bleaching treatment on pure-cotton knitted fabrics, wherein waste liquid is not discharged after the treatment is completed; (2) after the bleaching treatment is completed, adding deoxyenzyme to bleaching residual liquid, so that residual hydrogen peroxide is fully removed; (3) using the residual liquid for preparing dyeing liquid of reactive dye and dyeing the pure-cotton knitted fabrics, adding an alkaline agent for fixing color; (4) finally, washing, soap-boiling, washing and naturally drying the dyed fabrics. Two processes of low-temperature bleaching and reactive dyeing of pure-cotton knitted fabrics are performed in the same treatment bath in the treatment method provided by the invention, so as to not only shorten process flow and ensure dyeing performance of cotton fabrics, but also reuse the bleaching residual liquid, achieve apparent water conservation and energy conservation effects, and reduce pollution on environment from printing and dyeing waste liquid.