Drilling operation is very difficult in Shaling mining area in Laizhou of Shandong Province, where the drillability grade is 7～9, this typically complex formation is hard, brittle, loose, leakage and broken. Through a series of exploration and tests of the drilling fluid, flushing fluid formula was optimized to improve the drilling efficiency and reduce downhole accidents with safe production and high economic benefits. 117 holes have been completed in this mining area, the total drilling footage is up to 180795.10m with average depth of 1545.26m, and the deepest hole is 2117.70m.