Employing the theory of fluid mechanics, the authors present a theoretical model of the pressure leaked out from the leak valve in the system under super-pressure (i. e. the variational laws of pressure and flowrate in the system). By means of this model, the calculating results are in good agreement with the experimental data. A calculating formula for the fluid flow velocity and the temperature in the flow passages of the valve as well as the pipes in the leak valve are also offered. Finally, the destroyod mechanism of the valve is analysed and a calculating formula for estimating the magnitude of pressure due to scouring and cavitation is recommended.