Abstract: Aiming at the operation complex and low setting success ratio of convention casing window sidetracking tool, mechnical assembled casing window sidetracking tool was developed.The tool is including mill cone,deflector and anchor,the anchor setting,cutting window and fix window are operated by once going in the hole.Through theory account and strength check,Analysing and studying the structural design,material choosing and machining technics of the tool.The application in 15 wells at the field showed that the tool technics simple,performance stability,work efficiency high,has vast application prospect.