Abstract: In view of the collapsibility of borehole wall and coal reservoir pollution resulted from drilling fluid in Qinnan demonstration plot, and based on the resear chin 11 th Five-Year, the ultra low permeability drilling fluid agent and lubricant were optimized and evaluated through laboratory experiments, and the ultra low permea bility dilling fluid agent DL-1 and the environmentally friendly lubricant SD-302 were selected. Adding DL-1 with the mass fraction of 2% in bentonite slurry, and aging a t 120。C for 16 hours, invasion depth of coalbed is 7. 33 cm, which is superior to the performance of similar foreign products. In the common mass fraction range ( 1% ~ 3%) of lubricating agent SD-302, friction coefficient reduction rate is 72. 4% ~ 87. 7%. A new formula of hollow glass microsphere low density drilling fluid is developed using the DL-1 and SD-302. Compared with the original formula, the improved new formula of drilling fluid has a lower lubrication cofficient of 0. 041, which is benefici al to reduce the phenomenon of blocking or sticking. The new formula has better sealing effect on different permeability of reservoir, and has better anti collapse perfor mance and protecting effect on reservoir. The improved drilling fluid has been applied in two directional wells for field trial at Qinnan demonstration plot, which proved t he drilling fluid had good performances of preventing leakage and anti collapse, and ensured the smooth construction of drilling.