Abstract: For the fact that the growing number of high concrete dams built in valley, the problems existing in the dissipation of flow energy become more and more significant. Mechanism of energy dissipation of diffusion by sky-jump is discussed in this paper by means of turbulence theory and test. From the discussion such a conclusion can be arrived at that the essential of energy dissipation of diffusion are actions due to turbulence and due to entrained air. In addition, ways in which the effect of energy dissipation of diffusion can be improved are proposed.