Abstract: Among the reconstruct measurements of aged wells, sidetrack drilling is very prevalent, while during the drilling, it always meets different formation pressure coefficient, and one more layer of casing is applied, then the smaller bits may be put in use, the drilling operation difficulty increases. The Well of Lungu40C has met such an occasion. Sidetrack drilling generally uses milled window in casing because of worry about the size of the window will affect the next operation in the deep well of Tarim oil field. In order to save the procedures and the time, whipstock was put in use in this well. Having set the ?127 mm liner in ?152.4 mm hole and cemented successfully, then drilled to the designed well depth with the ?104.8 mm bit. Through the exploration of this well explain that the use of one time whipstock milled window in casing is feasible. Prevention of the occurrence of drilling pipe and tools accident is the key when drilling ?104.8 mm slim hole, and when the drilling pipe and tools fall off, pin tap might be the first selection of fishing tools.