The open and dynamic characteristics of cloud computing environment is easy to cause security problems, so security of the data resource and the privacy of user are facing severe challenges. According to the characteristics of dynamic user and data resources in cloud computing, a context and role based access control model was proposed. This model took context information and context restrict of cloud computing environment into account, and evaluated the user access request and the authorization policy in server, which could dynamically grant user's permission. The implementation process of cloud users accessing the resource were given, and the analysis and comparison further illuminated that the model has more advantages in the aspect of access control. This scheme can not only reduce the complexity of management, but also limit the privileges of cloud service providers, so it can effectively ensure the safety of cloud resources.