Wireless Smart Utility Network (Wi-SUN) has been standardized to provide Machine-to-Machine (M2M) communication in Low power and Lossy Networks (LLNs). Wi-SUN Field Area Network (FAN) is the specification that maintains a wireless multi-hop communication on the advanced smart city infrastructure. Wi-SUN FAN has adopted the Internet Protocol version 6 (IPv6) Routing Protocol for LLNs (RPL) which has been successfully optimized in fixed terminals. To establish the embedded smart city, mobile nodes are required to implement the RPL as well. Nevertheless, the RPL fails to choose the appropriate routing node when the terminal moves, resulting in high packet loss. To improve the transmission performance, the system simulator of Wi-SUN FAN with the Expected Transmission Count (ETX) based algorithm is firstly developed to evaluate the end-to-end transmission success rate via multi-hop transmission and the simulation result clarifies that the critical factor causing low transmission performance is the unsmooth handover of the receiver node at first hop. Then, a dynamic routing algorithm based on ETX transition is proposed and the simulation result finally assures the smooth handover, resulting in the improved transmission performance.