The invention discloses a low-temperature bleaching and active dyeing one-bath treatment method for purified cotton knitted fabrics, and belongs to the technical field of textile printing and dyeing. The method adopts the following processes: (1) performing low-temperature active bleaching treatment on the purified cotton knitted fabrics, wherein waste liquid does not need to be drained after the treatment is completed; (2) after the bleaching treatment is completed, adding deoxidizing enzyme into residual bleaching liquid to make residual hydrogen peroxide fully removed; (3) preparing active dyeing liquid by utilizing the residual liquid, dyeing the purified cotton knitted fabrics, and adding an alkaline agent for fixation; (4) finally, washing, soaping, washing and naturally air-drying the dyed fabrics. According to the low-temperature bleaching and active dyeing one-bath treatment method for the purified cotton knitted fabrics, the two processes of low-temperature bleaching and active dyeing for the purified cotton knitted fabrics are carried out in the same treatment bath, so the process flow can be shortened, the dyeing performance of cotton knitted fabrics is ensured, the residual bleaching liquid is recycled, the water saving and energy saving effects are obvious, and the environmental pollution caused by waste printing and dyeing liquid is reduced.