Abstract: Well AT-6X is the first slim-hole sidetracking well in the ultradeep and ultrahigh temperature buried hill of Huabei Oilfield, and in the process of its construction, it underwent several technical difficulties, such as poor rock drillability, difficult separation of new and old holes, large friction torque and high well temperature. In order to solve these problems, a series of studies were carried out from the aspects of sidetracking tool selection, bit feeding mode selection, design of the drilling fluid system of high temperature and low solid phase, and high temperature MWD instrument selection. In this paper, the mechanics model of sidetracking tool was established to analyze the side forces of drilling tools, and PDC bit +straight screw rod +2.5° bent sub was taken as the optimal sidetracking tool. The azimuth correction system was designed based on the indirect azimuth correction method to improve the precision of hole trajectory. The semi-active and semi-automatic bit feeding mode was adopted to increase the sidetracking efficiency. The drilling fluid system of high temperature and low solid phase was selected to improve the stability under high temperature. These techniques were applied in Well AT-6X. Its sidetracking operation is completed successfully by adjusting the tool face angle into 140° and adopting the semi-active and semi-automatic bit feeding mode, and its total depth is 6 059 m. The application of these techniques effectively solves the sidetracking difficulties in Well AT-6X, which provides the experience and reference for the sidetracking of ultradeep wells under the working conditions of high temperature.