Abstract: The KPAM polymer drilling fluid presently used in horizontal drilling in Yanchang Oilfield has low inhibitive capacity and poor rheology, resulting in frequent pipe sticking and inability to exert weight on bit during drilling. To improve the performance of the KPAM polymer drilling fluid, a cationic emulsion polymer DS-301 and a emulsified paraffin RHJ-1 have been used to replace the main polymers used. DS-301 is a high molecular weight (more than 6,000,000) cationic polymer inhibitor with multiple cationic groups, and is thus able to effectively viscosify the continuous phase of the drilling fluid. RHJ-1 has superior lubricating and inhibitive capacities, synergistically working with DS-301 to enhance the chemical inhibitive capacity of the drilling fluid. Based on laboratory experiments, a drilling fluid formulation containing 0.3% DS-301 and 2.0% RHJ-1 was selected. The new drilling fluid formulation, after hot rolling for 16 h at 100℃, had plastic viscosity (PV) of 25 mPa·s, yield point (YP) of 12.5 Pa, coefficient of friction of 0.233, and percent recovery of shale cuttings of 92.8%, better than KPAM polymer drilling fluid in rheology and inhibitive capacity. Pipe sticking has been greatly mitigated since the use of this new drilling fluid. 1% extreme pressure lubricant JM-1 was added in the drilling fluid to improve its lubricating performance and solve the problem of being unable to exert weight on bit in horizontal drilling, and the coefficient of friction was further reduced to 0.137. In field application, the filtration rates of the drilling fluid in three wells were controlled below 5 mL, and the ratios of YP/PV at about 0.48 Pa/(mPa·s). Cuttings carrying capacity of the drilling fluid was improved, and borehole wall stabilized. No downhole troubles have ever been encountered during drilling, and higher ROP was achieved.