Abstract: Window milling sidetracking horizontal well in slim hole is difficult than regular sidetracking well, even though there are some practical sidetracking slim holes in many oilfield, and some researchers discuss its trajectory design, window milling technology, onsite practicing, few papers report the trajectory controlling method. This paper discuss the bottom hole prediction, pre-drilling design, BHA selection on the basis of factors that affects trajectory controlling. Some theoretical and practical methods are summed up in the end.