Abstract: Continuous mass production of wells in Changbei Block has led to formation pressure attenuation, differential pressure increase and formation damage by drilling fluids. Therefore, the damage mechanism of reservoir has been studied. After doing so, the G311 water-locking removal agent and other agents were implemented, so as to form a clay-free drilling fluid with anti-water locking and lower damage performance, and it is suitable for the Changbei Block depleted reservoir. Compared with the clay-free low damage drilling fluid, its filtrate has a surface tension reduction rate of 77.8%, a linear expansion rate of 22.6%, and a core damage rate of lower than 15.0%. It has the advantage of water-locking removal, water sensitivity inhibition and remarkable reservoir protection effect. The new drilling fluid has been tested in 2 wells of the Changbei Phase II project, and no downhole failure occurred during the drilling process. The trip operation was smooth, and the wellbore maintained a well cleaned state. Among them, the average ROP of Well CX-5 increased by 12.1%, the open hole completion and direct gas lifting production were used in this well, and the production reached 70×10⁴ m³/d, higher than the expected gas production. The research results showed that the clay-free drilling fluid with anti-water locking and lower damage performance could meet the requirement of drilling safety and reservoir protection, suitable for long horizontal section drilling in pressure attenuated formation of Changbei Phase II.