Removable memory device (RMD) is one of main way for propagating computer virus. In this paper, a dynamic propagation model of RMD-virus with varying population size is discussed. Unlike other computer virus propagation models, the proposed model mainly considers the RMD-virus propagation between host and RMD, which is embodied by introducing the RMD state and new propagation rate. Furthermore, to control the RMD-virus, three threshold parameters are obtained. The simulation results show that the proposed model can serve as a basis for understanding and simulating RMD-virus.