In the second-phase project of Chengdu Wastewater Treatment Plant, the sludge digesters have an inner diameter of 27 m and a height of 20 m, with four units in total, completed and put into operation by the end of 1999. In the design of this structure, a circumferential unbonded prestressing structural scheme was adopted to meet the high circumferential prestress requirement of large-diameter, high-water-level digesters, achieving favorable economic and technical indicators. This paper briefly introduces the design and construction of the prestressed part of the digester.