Abstract: In order to solve the problem of back pressure during directional drilling,the mechanism by which it occurs was investigated by analyzing the force on the bottom hole assembly (BHA).Then,the main factors affecting back pressure were explored,and accordingly primary technical countermeasures were proposed.It was shown that the back pressure was mainly caused by friction resistance between drillstrings and borehole wall,and it was dominantly affected by wellbore trajectory,downhole cleaning,BHA,drilling fluid lubricity and formation types.It was proposed to control the back pressure by optimizing wellbore trajectory and BHA,increasing downhole cleaning and drilling fluid lubricity,and adopting down hole tools which could reduce back pressure.And finally,the application of back pressure controlling measures in Wells D15-X14 and Y123-5HF were introduced.It was indicated that these technical countermeasures for back pressure control were more effective.Based on these countermeasures,the frequency and degree of back pressure decreased significanlty and the rate of penetration(ROP)also increased.The research results provided the reference and basis for the directional drilling.