Abstract: Weizhou 11-1N-A12 is a delineation and injection well of Southwest China Sea Oilfield, a new type of hydraulic milling window cutting tool made by Weatherford Corporation was used to do sidetracking in 9-5/8" casing of this well. Directional drilling, hanging, window cutting, and window mending was achieved in one drilling and drilled in new formation with 14.12 m. Components and preponderance of this new tool were described, how to control the drilling parameters during the process of window cutting was discussed, and some experiences such as setting and disposing of the tool, orientation, and window quality control were summarized, which can be provided as technical reference for window cutting of similar wells.