With the further development of oilfield exploration, there are more and more abandoned wells and shutdown wells. Casing cutting window sidetracking drilling technology is an effective way to realize tertiary and secondary oil recovery, by which old wells can be made full use and drilling cost can be reduced to improve economic benefit and can effectively perfected the development of well pattern. The paper introduced window cutting and the directional construction of a φ139.7mm casing cutting window sidetracking drilling for a horizontal well.