There is a large section of shale formation in Iraq Rumaila oilfield, spalling and caving-in happen easily. The sandstone reservoir is thick and permeable, which is prone to leakage and generating slim hole. In the reservoir with good permeability, leakage and slim hole are often met. From the simple pursuit of the downhole stability at the very beginning to that the borehole wall stability, reservoir protection and lower friction in S & J shape wells are then taken into account, today's no-solid KCl polymer drilling fluid system is formed. The paper describes the formation of this drilling fluid system and the application effect in the laboratory experiments and the field application in detail.