Abstract: The standard of river diversion during construction is one of important problems involving safe and economic design scheme of hydroelectric project. After analysing the stochastic characteristics of river diversion process during construction, various uncertain factors including a stochastic initial condition, a stochastic coefficient and a stochastic input term are introduced into calculating program of level profiles before cofferdam, a stochastic differential equation is established. By using the Fokker-Planck equation,the probability density distributions of the level profiles before cofferdam are solved.Based on discussing the risk under different standards and design schemes of the river diversion during construction, the risk decision method is applied, a reasonable standard and design scheme of the river diversion for some project are suggested.