Aiming at the problem of risk assessment of software as a service （SaaS）， an assessment framework is proposed and the assessment method is given. The key elements of each evaluation factor are pointed out， and the membership matrix is constructed by expert scoring， the objective weight of factors is determined by entropy method， and finally the aggregated risk is synthesized. A case is provided which shows that the risk assessment principle and our calculation method are feasible. This work provides theoretical and methodological support to SaaS service providers， SaaS adoption businesses and other stakeholders in assessing SaaS risk.