Cloud computing brings efficiency improvement on resource utilization nd other benefits such as on-demand service provisioning, location independence and biquitous access, elastic resource pooling, pay as usage pricing mode, etc. However, t also introduces new security issues because the data management and ownership re separated, and the management is operated on a virtualized platform. In this paper, novel dynamic secure interconnection (DSI) mechanism is proposed to isolate he cloud computing system into a couple of dynamic virtual trust zones with different ecurity policies implemented for different customers so as to enhance security. xperimental results are presented to demonstrate the feasibility and effectiveness of he DSI mechanism