Abstract: Sidetrack with cement plug is a drilling technology that dills a new wellbore after placement of cement plug in the original open hole. Due to unfavorable oil indications of the reservoir encountered by ?215.9 mm wellbore, a horizontal development well located in Wenchang Oilfield was sidetracked after placing cement plug in the openhole. In order to overcome such difficulties such as large depth of sidetrack point, high formation strength, large heaving of semi-submersible drilling platform, etc., this sidetracking operation was successfully completed by using short gage PDC bit with strong side attacking ability and rotating steering tool Power Drive to dill the sidetrack hole, using sliding groove, creating steps and controlled drilling to control the well trajectory, and using cuttings logging and near bit inclination to monitor the realtime sidetracking job. The successful execution of this sidetracking job, the smooth well trajectory and the successful runing of subsequent sand-control screens, provide useful references for the sidetracking operations for horizontal open hole from semi-submersible drilling platform in the future.