Abstract: A risk evaluation method based on the cloud theory was put forward for evaluating project risk quantitatively. Cloud theory was proved to be an excellent mathematical tool for dealing with uncertain and vague description of objects, and it can realize the transforms between qualitative concepts and their quantitative expression . Based on the cloud theory, the system performance model and the cloud model of the risk assessment were proposed .A new way for project risk assessment was given and it can solve the transition problem between qualitative and quantitative in risk assessment. The project risk model was constructed by analyzing the causal relation between risk and risk factors, and the research way of project risk model was expanded in our country of a period of economy, and the applied value of the system of risk assessment was illustrated by practical application.