Abstract: Well Chashen-1 is a key exploratory vertical well deployed in Qian North area, with depth of 5 500 m. Serious wellbore sloughing and circulation loss take place in upper zone of Qingshankou of the well. The lower formations have higher temperature, and the maximum is 180 ℃ in the bottom hole. Through indoor experiment study, complex film aluminum based drilling fluid was taken for the upper formations, and for the lower formations, air drilling was for the former, and unbalanced drilling was for the later, with oil in water drilling fluid. The application showed that the drilling fluid effectively solved the difficulties of mudstone sloughing, circulation loss, and instability under high temperature, which met the need of the drilling operation.