Abstract: Aiming at the problem of imprecise and inefficient user role assignment in the role-based access control model in the cloud environment, the role-based access control model based on attribute and trust is designed. and A user-role assignment optimization algorithm based on this model is proposed, the algorithm combines the minimum role set searching and the attribute-based role filtering, by deleting the role activation in the hybrid role hierarchy and Role inheritance, get the smallest role set, through the user attributes, environmental attributes, trust value filter to get the best role set, and accurate match to the user. Simulation results show that this algorithm can effectively reduce the number of roles in the system, shorten the assignment time of user roles and improve the accuracy of user-role matching in the presence of a large number of user role assignments in the cloud environment.