With the rapid development of cloud computing, more and more businesses and individuals put there storage and computing needs into clouds. But as people have no more effective solutions for the security problems, the use and development are limited when getting data across the clouds. In this paper, we present a Kerberos-based authentication mechanism of crossing the clouds in hybrid cloud services. In this mechanism the Cloudterminal get to the certification with the Private-clouds directly. Issued by the tickets of enterprise Private-clouds Cloudterminal can get the access to the data in the Public clouds in a way based on authentication. In this mechanism there's no need to manage and issue certificates and the key management is simple and easy to control. This paper achieved this certification system, laid the foundation for secure authentication and access controlling when getting data across clouds.