Abstract: In the adjustment wells drilled in recent years in Block Suizhong, Bohai Oilfield, the percentage of highly deviated wells and horizontal wells have been increasing every year. This leads the formation pressure to deplete seriously, and the low molecular weight cationic polymer drilling fluid (JFC) is becoming unsuitable for the drilling operation because of the deficiencies of JFC in inhibitive capacity, rheology, contamination tolerance, and reservoir protection. To solve these problems, laboratory studies were conducted on the issues such as reservoir characteristics, the physical-chemical property of shales drilled, and the adaptability of drilling fluid to the formations penetrated. A drilling fluid additive, HAS, was selected through laboratory experiments to treat the drilling fluid presently in use. It was found that, compared with low molecular weight cationic polymers, polyamines and organic MMH etc., HAS was much stronger in inhibiting the hydration and swelling of clay minerals. Another additive PF-VIF was selected to replace the filter loss reducer RS-1 and XC, and the rheology and cuttings carrying capacity were improved. A combination of PAC-HV, PAC-LV and HAS can prevent the drilling fluid from becoming flocculated, and increase the solids tolerance of the drilling fluid. The use of plugging agents LPF and HTC together reduced the filtration rate of the drilling fluid, and enhanced the performance of the drilling fluid to protect the reservoir formations. In field application of this drilling fluid, drilling efficiency was greatly enhanced. The rate of back reaming in the conglomerate section below the Guantao Formation was increased by 30%, and drilling time was saved by 41.08 d. Wells drilled with this drilling fluid had oil production rates reached or exceeded the forecast.