A Computer-aided design (CAD) System for cyclic activated sludge system (CASS) was developed, which combined a new wastewater treatment technology with computer technology. It has important practical significance to enhance design quality, shorten the design cycle, and reduce the intensity of the work. With analyses of the process design of CASS, the algorithm analysis for the CAD system of CASS was completed. On platforms of Windows XP Pro SP2 operating system, the connectivity between the CAD system and AutoCAD was established, and then CASS automatic design and automatic engineering drawings were realized, using Visual Basic 6.0 as the development tool and ActiveX Automation as interface technology. An engineering case using the developed CAD system was demonstrated.