Cluster routing is an effective strategy to save energy of Wireless Sensor Networks (WSNs). The selection of cluster head is the key of cluster routing. However, the traditional cluster routing is to update the cluster-head in a fixed period without considering the residual energy of the cluster-head. Therefore, the Stable Election Protocol(SEP) is improved to propose a cluster-head update strategy based on multi-level energy threshold, denoted as I-SEP. The I-SEP route considers three types of nodes with different initial energies. According to the energy and proportion of the three types of nodes, the probability and threshold of them becoming cluster heads are calculated. At the same time, the energy of the cluster head is calculated in each round. Only when the energy of the cluster head is less than the energy threshold, the cluster head is updated; otherwise, the original cluster head remains, thus reducing the energy consumption caused by the replacement of the cluster head. Simulation results show that compared with SEP, the proposed I-SEP route effectively reduces the energy consumption and prolongs the network life.