Abstract: The properties of microfoam drilling fluid are quite unstably in the downhole environment of high temperature and high salt content. To solve this problem, a new type of microfoam drilling fluid was designed in this paper based on the idea of optimizing the molecular compositions of foam. It structurally consists of two parts, i.e., internal structure and external structure. The internal structure is a gas nucleus and the external structure is a film system which is composed of continuous phase surfactant, polymer processing agent and nanomaterial (SiO₂). Therefore, this design is named as the nucleus-shell structure of polymer nanometer assembly. The formula of microfoam drilling fluid is water + 0.4% AOS + 0.3% guar + (0.12%-0.18%) nano-SiO₂. And its microscopic properties were evaluated by means of electron microscope. It is indicated the performance of this microfoam system is stable. When the temperature is above 140 ℃, its foaming capacity is still good if it is added with 4%-8% KCl solution and 0.4%-0.8% CaCl₂ solution. It is demonstrated that the microfoam drilling fluid system with the nucleus-shell structure of polymer nanometer assembly is well resistant to high temperature and high salt content. To sum up, this design is of great significance to improving the performance of microfoam drilling fluid systems.