Abstract: Because cloud computing has the characters of virtualization, open, multi-tenant and elastification, cloud users and cloud objects always are changing dynamically, thus access control in cloud is more complex than traditional access control. The paper proposed Credible Usage Control Model in Cloud Computing (CUCONC),and based on the model given CUCONC reference monitor. CUCONC possesses Process continuity and attribute variability. By introducing credibility, CUCONC simplifies authorization policy,can change dynamically authorized rights,easy to grant or revoke rights. At the same time, the model enhances the manageability, flexibility and actionability.