Characteristics of supersonic cold flows over cavities were investigated experimentally and numerically, and the effects of cavities of different sizes on supersonic flow field were also analyzed. Results indicate that cavity flow structure has little relevance to L/D within the range of 5～9, and the ramp angle of the rear wall will not alter the overall structure of the cavity flow within the range of 300～600, but it can alter the development of the shear layer and the structure of the circumfluence in cavities.