A method for preparing drinking water from sewage treated on the basis of CASS-SSF (cyclic activated sludge system-sub surface flow) comprises the following steps: (1) alkali liquor with concentrationbeing 10-50 g/L is fed automatically to effluent treated by a CASS to regulate pH of the effluent, and first pretreated water is obtained; (2) a chemical feeding pump is controlled to start by an online pH meter and feeds an acidic material with concentration being 12-55 g/L to the first pretreated water automatically, and second pretreated water is obtained; (3) a flocculant is fed into the second pretreated water, solid-liquid separation is performed, active bottom sludge obtained after solid-liquid separation is regulated and collected, and a supernatant serves as third pretreated water; (4) the pretreated water is subjected to reverse osmosis treatment, and the drinking water is obtained. The method is simple, equipment investment is small, automatic operation can be realized, unmanned operation is realized, an operation mode is flexible, smaller area is occupied, higher adaptability is realized, the method is quite suitable for being popularized and applied to communities, schoolyards, industrial parks and the like, and conductivity of treated purified water reaches 8 mu s/cm which is better than that specified in bottled water drinking standards set by the Ministry of Health.