Abstract: To address the challenges of wellbore instability and difficulties in rock transport during deep coal bed drilling in the Ordos Basin, experiments such as rock component analysis and CT scanning were conducted on potential lithologies that might be encountered. It was found that the clay mineral content in coal gangue was over 28%, mainly composed of illite and montmorillonite. 96% of coal rock pores diameters ranged between 0.9 and 13.5 nm, indicating that the coal block has a mesoporous structure. Based on the characteristics of coal rock, a multi-functional drilling fluid system was developed by optimizing the plugging agents, inhibitors, and rheological parameters on the basis of the original brine drilling fluid. Comprehensive evaluations of plugging, inhibition, and rheology were carried out, and the sand loss of the drilling fluid in the sand disc was reduced by 2.6 mL, the rolling recovery rate of coal gangue was increased by 12.70%, and the triaxial compressive strength was increased by 6.52%. The drilling fluid system was applied in two wells on-site, confirming stable coal wellbores, smooth tripping and casing runs. The average wellbore enlargement rate of the third section in the construction wells decreased from 28.48% to 19.01%, and the reverse reaming tripping speed increased from 3.93 m/h to 9.41 m/h. The multi-functional drilling fluid system has solved the problems such as wellbore collapse and sticking in deep coalbed methane wells and provides a solution for other wells facing similar construction issues.