Because of existence of many uncertain factors in water environmental system,the model established on the basis of the physical simple does not reflect the uncertainty of water quality and water quality change. In order to compensate the limitations of deterministic model of water quality,the paper supposed oxygen coefficient, sedimentation coefficient and reaeration coefficient to obey the doubly truncated normal distribution hypothesis in water quality model according to the theory of stochastic differential equation,and set up one-dimensional COD model considering random effect.The model was used to risk analysis of COD exceed the standard.The paper calculated the probability distribution of COD in different condition. The result showed that when the COD is 16, 17, 18 mg/L, the risk probability over standard is about 0.3, 0.6,0.8.