Abstract: The organic salt drilling fluid of Weigh series has advantages of strong inhibition, high temperature resistance and low friction, and it has been the main drilling technology in complex situations at home and abroad. In this paper, the water activity of Weigh organic salt drilling fluid, the thickness of adsorption-diffusion double electrode layers of clay colloidal particles after the Weigh organic salt drilling fluid effect, and the layer space of montmorillonite crystal were measured in laboratory. Then, the inhibition mechanisms of Weigh organic salt drilling fluid were discussed from three aspects, i.e., activity, double electrode layer and ion exchanging lattice embedment. It has been successfully applied in two extended reach wells in the Beibu Gulf, western South China Sea. It provides the technical support for the development of similar oil fields in western South China Sea.