Seamless handoff issues including authentications have become considerable interest in wireless networks, which support pervasive environment users. Usually the active scanning and preauthentication methods are adopted to solve the disruption in seamless services. In this paper, we propose a new preauthentication scheme based on the fast channel switching and the power saving mode (PSM) in public wireless local area networks (LANs). We prove the effect of our proposed scheme using the simulation programming results. During the channel switching period (CPS), our proposed scheme performs multiple preauthentication with pre-scanned neighbor Access Points (APs). By using very short period of channel switching, our scheme doe not degrade Quality-of-Service (QoS) of its current applications. It also reduces the authentication latency in handoffs and improves the random mobility of users. In addition, our scheme only requires minor implementation changes on the client device driver, so it is apt for being deployed in current WLANs environment.