Comprehensive power grid risk assessments can effectively improve the management level and strengthen the security of power grid operation, to ensure reliable operation of the power system. This paper raises a comprehensive power grid risk assessment method based on cloud prediction model. First, the cloud theory is used in power transmission equipment failure prediction to build prediction rule based on cloud generator. On the basis of the prediction rule, a cloud-based power transmission equipment failure rate prediction model is built. And a power grid assessment system is raised based on utility theory, including over limited voltage index, limit transmission power index, potential cascading failure index, and system stability index. The assessment method could be used to estimate the comprehensive risk of power grid in order to scan the weakness in grid operation. Finally, the effectiveness and practicality of the power grid risk assessment method based on cloud prediction model is verified by an actual case.