Abstract: Fahaniu Block in Liaohe Oilfield is faced with serious mud making, prominent borehole wall instability, difficult rock carrying during coiled tubing drilling and frequent sticking.To solve these problems, a strong-inhibition water based drilling fluid system was developed after polyamine inhibitor, blocking agent and high-efficiency lubricant were selected.And then its performance was evaluated in laboratory.It is shown that this system is resistant to the temperature of 180 ℃, the salt of 20%, calcium of 5% and drilling cuttings of 25%.Its rolling recovery rate of cuttings is 98.7%, and core permeability recovery is more than 90%.It can block the fractures of 0.1-0.3 mm.And furthermore, its pressure bearing capacity and flowback resistance are higher and its lubricity is similar to that of oil based drilling fluid.When this strong-inhibition water based drilling fluid was applied in Well Fa 51-NEW 161C, the field test well of coiled tubing, its performance was stable and adjustable and drilling was improved with excellent sand carrying capacity, high rate of penetration (ROP), regular well diameter, and remarkable reservoir protection effect.In conclusion, this strong-inhibition water based drilling fluid system can meet the technical requirement of the coiled tubing drilling in the mud shale formation and it is worth popularizing and applying.