With the rapid development of Internet of Things (IoT) technology, the number of sensors in IoT is proliferating and the monitoring range is expanding. Wireless sensor networks (WSNs) in IoT are large-scale WSNs with fast changes in node energy due to long-distance communication, and node distribution due to the death of nodes. A clustering and routing algorithm for the fast changes (FC-CRAs) of large-scale WSNs in IoT is proposed. In the FC-CRA, the clusters are constructed according to the cluster radius which can dynamically adapt to the change in node energy and distribution. The intracluster routing is based on the path energy function to save and balance node energy. The intercluster routing is performed in an intercluster communication node set to avoid the premature death of near-BS nodes and ensure the continuity of data transmission. Compared with the other algorithms, the FC-CRA can improve the node lifetime, network throughput, and save the network energy in large-scale or sparse WSNs. The proposed algorithm can alleviate the “energy hole” problem and improve the reliability of data transmission, which is very important for some IoT application scenes with wide-area monitoring requirement and harsh or hazardous situations.