Abstract: This study derives formulas of the similarity between compressible gas flows and open-channel water flows through calculus transformation based on the basic relationships of thermodynamics and hydrodynamics, and obtains mapping relationships between similarity parameters, their application scope, and the limiting factors. We design a specific nozzle based on the method of characteristics, construct an open-channel flow test stand, and conduct tests on cavity flows in the open channel, using a high speed camera and fluctuating pressure sensors for measurements. The experimental results verify the similarity between compressible gas flows and open channel flows, and show that the cavity flow has a dominant pressure fluctuating frequency of 2.09 Hz when its Froude number is 2.0 and flow rate 11 m3/h. Thus, this study would help model supersonic gas cavity flows experimentally using a more convenient open-channel flow approach.