According to the characteristics of high formation pressure, active formation fluid and difficult construction for horizontal wells in 503 block of Daqing oilfield and by the laboratory experiments, the original low-viscosity water-in-oil drilling fluid system formula has been optimized so that when the upper limit of density reaches 200g/cm3, the controllable range of oil-water ratio is from 75∶25 to 90∶10, and the drilling fluid still has good rheology, electrical stability and sedimentation stability. By the application in well Chao42-P125, the construction requirements are met and downhole safety is ensured.