Wireless sensor technology plays an important role in the military, medical, and commercial fields nowadays. Wireless Body Area Network (WBAN) is a special application of the wireless sensor network in human health monitoring, through which patients can know their physical condition in real time and respond to emergencies on time. Data reliability, guaranteed by the trust of nodes in WBAN, is a prerequisite for the effective treatment of patients. Therefore, authenticating the sensor nodes and the sink nodes in WBAN is necessary. This paper proposes a lightweight Physical Unclonable Function (PUF)-based and cloud-assisted authentication mechanism for multi-hop body area networks, which compared with the star single-hop network, can enhance the adaptability to human motion and the integrity of data transmission. Such authentication mechanism can significantly reduce the storage overhead and resource loss in the data transmission process.