The underwater two-phase ramjet is one kind of the new style jet which is based on the development of the two phase bubbly flow throughout the Laval nozzle. The working mechanism of the ramjet is studied numerically by using CFD commercial software FLUENT 6.2. A steady flow, non-compressible air and non-exchanging heat are considered. When the velocity of the ship and the aeration angle is steady, and the aeration speed is supersonic, the inner thrust of the ramjet increases as the aeration speed increases. The inner thrust fluctuates in a wide range when the aeration speed increases to a very high speed. When the air is aerated only, the temperature of the air doesn’t have effect on the inner thrust almostly. While the gas aerated is composed of air and vapour, and the volume flux of the gas remains invariable, that the humidity of the gas increases causes a drop of the inner thrust of the underwater two-phase ramjet.