Air-breathing scramjet propulsion is the basis of air-breathing solar thermal propulsion technology. How to design an ideal air-inlet is crucial for the propulsion system. By taking advantage of the common method, the DSMC(direct simulation Monte Carlo) on rare-field gas dynamics research, was made to simulate two kinds of air-inlets, which obtained a lot of air parameters such as temperature, density, coefficient of flow and velocity. Then it selected a better air-inlet shape from comparison and treated the shape as a basic air-inlet shape in airbreathing solar thermal propulsion system. It lays a foundation for the design, computation, analysis and optimization on the whole system.