A risk evaluation index system is established for assessing investment projects from seven respects, such as the entrepreneur and team risk, product risk, technology risk, market risk, exit risk, policy and industry risks and regional influence, and a risk assessment approach based on the cloud model and Theil index is proposed. The evaluation indexes are transformed into cloud models, the weights of all indexes are calculated by using the cloud characteristics and Theil indexes, and on the basis of the weights, the cloud models are aggregated for deriving the comprehensive evaluation values of each alternative, and then the best alternative can be selected. Finally, the application of this method shows that it is feasible and effective.