Abstract: A mathematic model of the stochastic differential equation (SDE) is applied to describe and analyze the random phenomena and their laws in the reservoir flood regulation procedures. According to the analysis of a Wiener process characteristics of the reservoir storage, an Ito Equation with a stochastic input term and a stochastic initial condition for reservoir flood routing is derived. And, by using the Fokker-Planck Equation, the probability density distributions of reservoir level hydrograph in the flood regulation procedure are solved. The calculated results show that the mathematic model of SDE in reservoir routing is favourab 1e in understanding the laws of the stochastic flood regulation procedure and developing risk analysis for flood relief.