By referring to the requests of access control in the power dispatching automation system and the defects of the role-based access control model， a new credibility-based role access control model is proposed. It combines the user behavior trust with identity trust. With user login implied information captured in the process of user authentication， the uncertainty reasoning theory of artificial intelligence is used to calculate the users credibility， which then participates in the user authorization. The credibility active threshold to roles and permissions is set， and the user can not get their deserved permissions unless they can satisfy the credible active constraints for roles and permissions. A flexible system delegation can be realized to ensure a safe user access to system resource， making this model well applied to the existing power dispatching automation system.