Abstract: Wellbore collapse and reservoir damage occurred when clean brine and conventional polymer drilling fluids were used in drilling horizontal wells of No. 15 coal seam in Qinnan Area. In view of this, according to reservoir characteristics and challenges encountered during drilling, a guar gum drilling fluid and a bio-enzyme gel breaking fluid were developed. Specifically, the guar gum drilling fluid was formed by optimizing the dosage of guar gum, evaluating the salt tolerance, and integrating with other treatment agents. And the bio-enzyme gel breaking fluid was produced by optimizing types and dosage of bio-enzyme and cleanup additive. The laboratory test showed that the guar gum drilling fluid had good rheological and salt tolerance properties. It could greatly improve the compressive strength of coal and rock, and easily achieve gel breaking at low temperatures, with residues after breaking less than 300 mg/L.The permeability recovery rate of coal rock was more than 85%. In terms of the application of the guar gum drilling fluid in coalbed methane (CBM) horizontal wells in Qinnan Area, the fluid showed positive wellbore stability. In addition, it not only achieved gel breaking at low temperatures but also improved the daily productivity of a single well by more than 15% after integrating with the bio-enzyme gel breaking fluid, which indicated a favorable reservoir protection effect. The research shows that the guar gum drilling fluid can ensure smooth drilling of long horizontal sections in coal seams and achieve gel breaking at low temperatures after drilling, which provides a new reservoir protection method for drilling horizontal wells in fragile coal seams.