In view of the drilling technical difficulties of poor wellbore stability and high friction torque in the long horizontal section of Well Yang101h3-6， high-density oil-based drilling fluid was proposed as the solution. With incorporation of a new “Ⅰ” type main emulsifier， the formulation of high density oil-based drilling fluid was determined at 2.2g/cm³ with the composition of 2% primary emulsifier+3% co-emulsifier+3% wetting agent+4% filtration reducer+1% organic soil+3% CaO， oil-water ratio 80∶20（diesel oil∶25% CaCl₂ aqueous solution）. Evaluation on its performance showed that the system has strong inhibition， good anti-pollution ability， and stable performance； the field application showed that the high-density oil-based drilling fluid has good rheology， high demulsification voltage， low filtration at high temperature and high pressure， meeting the drilling needs for the long horizontal section of Well Yang101H3-6. During the drilling of the horizontal section of Well Yang101H3-6， the performance of the drilling fluid was stable， the borehole wall was stable， and the downhole was safe， leading to smooth completion of the well.