The proliferation of smart gadgets, appliances, mobile devices and sensors has enabled the construction of pervasive computing environment, transforming regular physical spaces into "Active Information Spaces". However, access control remains a major challenge for the deployment of this computing paradigm in real-life. A context access control model based on UCON_ABC model is presented in this paper, which not only focuses on authorization, but also introduces obligations, and conditions. And it includes pre-authentication, pre-condition, pre-obligation, active-right and passive-right sub-models. The operations of sub-models are presented. And the security, flexibility and usefulness of our proposed model are demonstrated by a typical pervasive computing scenario.