Abstract: In the Bohai Bay basin,because of the existence of thick active soft shale in sedimentary formation,downhole problems happen frequently and hydrocarbon reservoir is damaged in a certain degree when drill this shale interval using ordinary polymer drilling fluid system with poor encapsulating ability,weak formation sealing ability,and bad shear thinning property.Therefore,we developed clay free high inhibitive drilling fluid system.Proceeding from mechanism of additives in the drilling fluid system,the most superior combination of the construction is determined.Study results demonstrated that this drilling fluid system has high inhibitive ability,and cuttings roller recovery achieves 95.6% which is close to oil based mud.It has good lubricity,heat tolerance,potential resistance to cuttings contamination,potential resistance to CaCl₂ contamination,potential resistance to MgCl₂ contamination,as well as good reservoir protection performance,so the return permeability is able to achieve above 85%.The drilling fluid can meet the drilling demands of thick active soft shale complex formation and it has broad application prospect.