Abstract: Geological structure is complex in Weixi block. Especially the mudstone intervals in the 2^nd segment of Weizhou Formation and 2^nd segment of Liushagang Formation are characterized by well-developed faults, micro fractures and strong sensitivity. During the drilling operation, collapse, drilling mud leakage and even sticking accidents is easily happened. Based on the analysis and study on the collapse mechanism in the hard and brittle mudstone in Weixi, KCl-Polyglycol drilling fluid system, a strong inhibition and plugging drilling fluid, was developed and used in the drilling operations of Well H2, Well W4 and Well W5 successively. Field application shown the drilling fluid system has good inhibition and seal performance, which guaranteed wellbore stability and downhole safety. The application of KCl-Polyglycol drilling fluid system ensures the wellbore stability and also contributes to the success of Well W4 from high production of more than one thousand tons of oil and gas per day was tested.