Abstract: In this paper, a mechanism of the evolution of unstable energy in the hail storm is given. It is shown that the evolution of unstable energy may be the result of the nonlinear propagation of energy in z-t plane and that the vertical transport in the cumulus convection is the propagation velocity of the wave.It is shown by the actual observational data that the evolution of unstable energy in a hail storm is in good agreement with the result given by the theory.