Abstract: The window cutting trace and window shape have greater effect on the coiled tubing sidetracking quality. At present, there is no specific study and special commercial engineering software on the prediction of window cutting trace of coiled tubing sidetracking. In this paper, the whipstock based window cutting in the case of two-milling-show bottom hole assembly (BHA) was studies. The model for predicting the window cutting trace of coiled tubing tracking in one-dimensional wellbore was established by means of beam-column method under the constraint of stall free motor and valid force. Then, based on the movement trace of the milling shoe, the contour shape of the window was depicted by means of point-by-point method. Finally, calculation and analysis were carried out by taking the approximately actual engineering data as the example. It is indicated that the drilling trace and casing window quality predicted under certain constraints are more accordant with field experience and cognitions. This research method and conclusion can be used as one means or important reference for assisting the design of construction schemes, e.g. construction parameter and window drilling tool selection under one-dimensional casing program, and can also provide the guidance for the extension studies on the casing window of coiled tubing tracking under two-dimensional (continuous deflection section) and three-dimensional casing program.