Abstract: Technical issues such as difficult wellbore trajectory control, low rate of drilling in formation, low ROP (rate of penetration), wellbore collapse, and circulation loss are easily encountered in the drilling of horizontal wells with ultra-long horizontal section and slim hole in Changqing Oilfield. In light of this, difficulties in drilling technologies were analyzed in this study and some key technologies were investigated, including intelligent wellbore trajectory control based on rotary steering, near-bit azimuthal gamma imaging, and engineering parameter monitoring. Moreover, bit selection was optimized and the application of sealing water-base drilling fluid by a formed nanometer film was studied. As a result, drilling technologies for horizontal wells with ultra-long horizontal section and slim hole in Changqing Oilfield were developed, which can enable accurate wellbore trajectory control, drilling ratio increase and ROP enhancement, and can ensure downhole safety. The rotary steering technology and related supporting technologies were applied to Well Tao XX in Changqing Oilfield, and drilling was completed safely and efficiently in a 4 466 m horizontal section at a depth of 8 008 m, with the drilling ratio of 96.6%. This created a new drilling record as the deepest well in Changqing Oilfield and the longest horizontal section on land in the Asia-Pacific region. Considering the good field application effect, the drilling technologies for horizontal wells with ultra-long horizontal section and slim hole in Changqing Oilfield are worthy of wide application.