In the oilfield of western South China Sea, the geological situation is complicated in Beibuwan basin reservoir section with the interlayer, the mudstone is easy to be hydrated, the drilling fluid system maintenance is difficult, and it is hard to give consideration to both borehole stability and reservoir protection. Take WZ6-9 and 6-10 oilfields as the examples, the paper introduces the geological situation in Beibuwan basin reservoir section and the difficult points in drilling fluid technology; also presents a new kind of solid free organic salt anti-sloughing drilling fluid system and the results of experimental evaluation. This drilling fluid system is effective for solving the sand-shale interbed borehole instability in complex reservoir section of Beibuwan basin, preventing the pollution of the reservoir and reducing the incidence of downhole accident. This technology provides a powerful technical support for the petroleum exploration and development in Beibuwan basin.