The Shunbei-52A well is an exploratory well deployed in the southern part of the 5# fault zone in the Shunbei Oil and Gas Field， and is difficult to complete with high drilling risks due to complex Silurian formation encountered. A salt water layer is present in the Silurian stratum； thus， the density window is narrow， and leakage and collapse may happen concurrently； the formation has developed fissures with high risk of leakage； there are large intervals of mudstone with strong water sensitivity， and the wellbore is easy to lose stability. Therefore， the KCl-NaCl composite under-saturated brine drilling fluid system was used for the Shunbei-52A well to enhance the ability to resist salt water intrusion； inhibitors such as polyamines were added to work together to inhibit hydration swelling of the mudstone， and enhance the inhibition of the system； in addition， the plugging agent and the plugging-while drilling material were used to prevent lost circulation， and enhance the plugging performance of the system. Field use showed that the composite under-saturated brine drilling fluid system not only reduces the risk of lost circulation， but also maintains the wellbore stability of the Silurian formation well. It enables safe and quick drilling of the Shunbei-52A well in the Shunbei Oil and Gas Field， providing valuable experience and technical support for the drilling operations in the Silurian strata in the southern part of the 5# fault.