Abstract: The marine gas pools in Yuanba region are buried deeply, and the major reservoirs have a vertical depth of over 6 500 m. During drilling of ultra-deep horizontal wells in this region, there are difficulties like high temperature and high pressure, complex formation lithologies, poor stability of measuring tools and difficulty in wellbore trajectory control, etc., so higher requirement is set for drilling technology for horizontal wells. First, analysis was performed to the difficulties in drilling operation in Yuanba Gasfield; through implementing the anti-deviation and fast drilling technology used in vertical wells, sidetracking technique for ultra-deep hard formations, efficient control technology for wellbore trajectory in building-up section and horizontal section, as well as the development of HTHP SLBF HT175 MWD tool, the MWD and control technology has been developed for ultra-deep horizontal wells in Yuanba Gas Field, which has greatly improved the capability of drilling of ultra-deep horizontal wells in Yuanba Gasfield. After SLBF HT175 MWD tool was used in four ultra-deep horizontal wells in Yuanba Gasfield, it was validated on site in terms of temperature resistance performance, pressure resistance performance, measuring accuracy and overal reliability, capable of meeting the requirement of MWD in ultra-deep horizontal wells in domestic oil/gas fields.