The invention relates to a one-bath-one-step energy-saving dyeing method for bleaching and dyeing a polyester-mixed cotton cloth. The method breaks a traditional bleaching and dyeing two-bath-two-step or one-bath-two-step technology, can save the bleaching time and decrease the energy consumption, and has a good cloth cover effect due to the reduction of the time; color fastness is consistent with that of a general technology; compared with the general technology, the method has the characteristics of being simple in processes, short in processing process, consistent in whiteness and consistent in color and color tone; the requirements on energy conservation and emission reduction are met; the method has an obvious effect after being applied to the bleaching and dyeing one-bath-one-step dyeing of bright-color or light-color polyester-mixed cotton cloth.