Abstract: Well Yuanba 103H, located in Bazhong structure of eastern Sichuan basin, is a demonstration ultra-deep horizontal well deployed by Sinopec South-Western Branch. Colloid stabilizers, filtration control agents and lubricants were evaluated and selected in labs, and a type of polymer drilling fluid is developed. Throughout the fifth interval of horizontal section drilling in Well Yuanba 103H, the drilling fluid kept working smooth, owing to: 1) appropriate density lowered the risk of pipe-sticking; 2) MBT between 2% and 2.5% guaranteed the high temperature stability; 3) synergy of efficient lubricants rendered the mud with good lubricity; 4) weak turbulent flow pattern kept the hole clean in annular space with large size and low flow rate. Application results showed that, this polymer drilling fluid system has good temperature resistance, contamination tolerance and lubricant effects.