Abstract: The project of drilling slim-hole sidetrack horizontal wells is conducted in Block G104-5 of Jidong Oilfield to improve oil recovery and reduce production cost.Well GCP105-5 is the first slim-hole sidetrack horizontal well drilled in this region.It is difficult to carry out sidetrack drilling operation due to the small borehole diameter,big change of curvature,complex strata in the sidetrack drilling section,and few reference data.During the construction of the project,the casing sidetracking spot is fixed in the cementing section of good quality according to the design process.Firstly,the low-speed milling casing is lightly pressed.Then the drilling pressure and rotation are increased gradually when iron filings come out from the wellhead.In the course of sidetracking,drilling bit types are optimized,flexible antisticking and reversed bottomhole assemblies are applied,KCl-based organic positive gel drilling fluid is used,four-step solids control and drifting operation are carried out.During cementing,the cement slurry density is kept uniform and the displacement is maintained stable.Well GCP105-5 is drilled successfully and the goal of stable borehole,good cementing effect,and no accidents is achieved,providing valuable experience for the operation of drilling the sidetrack horizontal wells in the area.