Abstract: Currently, CT sidetracking processes frequently suffer from reverse rotation of power tools, insufficiency in cutting powers and other key issues. Accordingly, it was proposed to mill BHA for windowing of CT sidetracking operations. In addition to highlighting the working principles of the BHA, mechanical models for vertical and vertical deformations have been constructed. Configurations of sidetracking windows have been designed and mechanical properties of milling shoes have been analyzed. Furthermore, impacts of drilling pressures, lengths of power tools, lengths and rigidity of flexible nipples and other factors on mechanical behaviors have been reviewed. Calculation results showed that lateral forces might linearly increase with increases in the eccentric displacement of milling, while no change could be observed in lateral force and rotation angle during milling along the vertical line of the deflecting surface. Lateral force increased from drop tendency to buildup tendency with increases in WOBs, whereas lateral forces decreased to minimum and then increased with increases in lengths of downhole motors. At the same time, lateral force decreased from buildup tendency to drop tendency with increase of length of flexible nipples. Lateral force increased with the decrease of bending rigidity of flexible nipples. The variations had no major impact on rotation angles. Research results showed mechanical properties of milling BHA deployed for sidetracking operations could be effectively regulated by adjusting drilling pressures, sizes of milling BHA and other parameters. In this way, rhw lateral forces of milling could be managed efficiently. Relevant research conclusions may provide necessary guidance for drilling operations on site.