Based on an open water tunnel, an experimental system for the coupling of ventilated cavity and supersonic jets was constructed. Four variable length experimental models were designed, focusing on the influence of the relative position of the jet and the ventilated cavity on the interaction mechanism. For different experimental models, the ventilation flow and jet flow were changed to obtain the dynamic process of the evolution of the cavity interface under different working conditions. Experiment observed three distinct mechanisms, which were divided into the following types:the jet fully inflated the cavity and the jet partially deflated the cavity; the jet fully deflated the cavity. By combining the experimental results and Paryshev′s theory, the calculation model of the cavity size and critical transition conditions for different mechanism were further constructed. When Ｊ≤0.1, the cavitation length can be described by the classic cavity empirical formula; When 0.1<Ｊ<1.5, the cavitation length is divided into two parts:the model length and the tail cavity length, and the length of the closed cavity from the nozzle outlet is only related to the parameter Ｊ; When Ｊ≥1.5, the ventilated cavity closes at the tail of the nozzle, the cavity length is approximately equal to the model length.