In order to reveal the influence of gravity effect and tunnel blocking effect on the minimum ventilation cavitation number. In this paper, based on the two-fluid multiphase flow and SST (Shear Stress Transport) turbulence models considering gas-water interaction, the three-dimensional ventilated cavity simulation model is established. The relations between minimum ventilation cavitation number and its influencing factors are analyzed respectively in unbounded flow field and water tunnel. The simulations show that minimum cavitation number decreases with the increase of Froude number in unbounded flow field in the form of a power function.In water tunnel the minimum cavitation number increases with the blockage ratio at the same Froude number. The change range of minimum cavitation number decreases with the increase of the blockage ratio at the same Froude number range.