The invention discloses a method for treating sludge by using a composite microbial agent, and belongs to the technical field of sewage treatment. The method comprises the following steps: step 1, calculating an amount of the composite microbial agent required to be added according to an amount of water to be treated in a feeding tank; step 2, accurately weighing the composite microbial agent including bacillus, saccharomycetes, actinomycetes, fungi, photosynthetic bacteria, lactic acid bacteria, lactobacillus acidophilus, lactobacillus casei and aspergillus niger; step 3, putting the composite microbial agent into the feeding tank, starting timing, and carrying out a reaction for a period of time; and step 4, in the reaction process, paying attention to observing a water temperature, pH value and sewage COD concentration in the feeding tank. According to the technical scheme provided by the invention, the operation is easy and convenient, the microbial composition and function of activated sludge are changed by directly adding the microbial agent, an original treatment process and operation conditions are not changed, the adaptability is better, the costs of the microbial agent are low, the treatment effect is good, and the method is suitable for application and promotion.