The invention discloses an activated sludge treatment method for wastewater. The method comprises the following steps that 1, under the anoxic treatment condition, the wastewater makes contact with a material containing activated sludge, and anoxic treatment on the wastewater is performed; 2, aerobiotic treatment is performed on a mixture obtained after the contact; 3, part of a mixed material obtained through aerobiotic treatment is returned to the step 1 to serve as part of the material containing the activated sludge, wherein the particle size of at least 60 wt% of the activated sludge in the mixed material which is obtained through aerobiotic treatment and returned to the step 1 is not larger than 1 mm. The invention further provides an activated sludge treatment system for the wastewater. According to the activated sludge treatment method and system for the wastewater, the degradation and nitrification efficiency of organic matter difficult to degrade in the wastewater is promoted, and agentia feeding and O pool aeration strengthening are not needed; the advantages of being low in operation cost and free of by-products are achieved.