USB mobile storage devices are widely used to transfer and exchange data for their small size and large capacity. These features provide challenges for us to protect our confidential information because thieves can take secrets away by USB storage devices easily. At present, there are many studies on how to protect confidential data on USB storage devices. Most of these studies are based on application layer or operating system layer. When there are malicious codes on operating system, the operations protecting confidential information can be easily bypassed by attackers. In this paper, we present a USB devices access control system which is implemented on a thin hypervisor. The thin hypervisor is transparent to OS, which can guarantee that the security of the system is independent of OS, so that the system can be more secure.