Abstract: In view of the complicated geologic conditions and incidents which might occur at BHW01 well, such as channeling and leakage, the organic salt drilling fluid system formula is developed, through indoor experiments and formula optimization. Field application shows that the drilling fluid system features in high solid-phase capacity, good inhibition, and strong anti-collapse capability, as well as better rheological property in the case of high density, which makes it easier to be maintained and adjusted. This could be valuable reference to the drilling of horizontal wells in Santai Oilfield and high density drilling fluid technology of horizontal wells. Besides, underbalanced drilling is fulfilled at the third-spud horizontal segment of BHW01 well.