Today, the problem of trusting cloud computing is a paramount concern for most enterprises. Whether user behavior is trusted and how to evaluate user's behavior trust are important research contents in cloud computing. This paper suggests an adaptive access algorithm by introducing the trust into cloud computing to decide the access control to the resources using an improved RBAC technique to solve more complex and difficult problems in the cloud computing environment. And the proposed model determines dynamically security level and access control for the common resources. Therefore, it is supposed to provide appropriate security services according to the dynamic changes of the common resources. This access control model bases on trust determines whether the user has the right to get access to the resource by dynamically authorizing. And this mechanism can control the user's malicious behavior effectively.