In order to prevent the possible leakage of user's sensitive information, it is necessary to delete them securely before USB flash disks are exposed to an untrusted environment. But it is impossible for ordinary users to change functions in the hardware layer of USB flash disks. In order to perform a userlevel secure deletion for USB flash disks, we analyzed data remnant for both logical layer and physical layer after several logical overwriting are applied to a USB flash disk. According to the analysis, a novel approach, called USRM (USB flash disk Secure ReMove), is presented in this paper. First, USRM tries to create junk files quickly to reduce the size of free space. Second, USRM removes target files and related information. Finally, USRM overwrites free space at the logical layer in order to force controller to clean any possible data remnant at the physical layer. Experimental results show that USRM can delete target files, quickly and securely, while comparing with another two secure deleting tools.