Abstract: Well slough is more likely to happen while drilling slant holes in Chaoyang Gou oil field of Daqing,because of the overlying mud making formation and the underlying slabbing formation. In this case,the drilling fluid should be good in slough prevention,lubrication and cuttings carring. The No.Ⅱ Drilling Co. of Daqing applied the amphion polymer drilling fluid system based on FA-578 and XY-38,and the results were successful. Field tests were conducted in 3 wells and the following was proved:the cuttings were packed well,mud making got controled,the ability of carring cuttings was high,triping in/out were easy,drilling sticking and well slough were avoided. It is stable and convenient to maintain and handle. The penetration rate was increased by 4m/h compaired with the wells of the same type,and it was shortened 4.38 days of the completion period.