Abstract: Existing solid-free drilling fluids may have low temperature resistant performance, but can not satisfy the demands for drilling of horizontal wells in a high temperature regime. To solve this problem, a solid-free high temperature resistant drilling fluid has been developed, taking a zwitterionic hydrophobic associated polymer (PL-5) as the main treatment agent. Its rheological property, filtration loss, suspension stability, inhibition and reservoir protection effectiveness were indoor tested and evaluated, and the microstructure of polymer in liquid drilling fluid was observed by means of atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM). Test results showed that the newly developed drilling fluid could maintain the desirable rheological property, filtration property and suspension stability after aging at the temperature of 160℃. The first cuttings rolling recovery rate at an upper formation was 60.1%, and later 87.2% at a lower formation. The new drilling fluid can effectively inhibit the hydration of mud shale, and the permeability recovery rate of reservoir core is up to 82.0%. The application effect in three high-temperature horizontal wells of DF1-1 Gas Field showed that the solid-free drilling fluid not only exhibited desirable property of rheology and filtration as well as suspension stability, but also had a good effect for reservoir protection and raising the drilling rate, which can meet the demands of drilling horizontal wells at high temperatures.