In this paper, we propose a variable packet length adaptive MAC Protocol (VPLA-MAC) for wireless sensor networks. VPLA-MAC is designed for clustering hierarchy networks based on the routing information collected from sensor nodes. A sensor node in cluster sends packet length and routing information to cluster head node, this cluster head node calculates the cycle length and selects nodes for data transmission, then information are broadcast via a sync packet. Thus, the nodes in the cluster can formulate cycle length based on packet length, while the unrelated nodes can go to sleep to save energy. The simulation results based on NS-2 show the efficiency of our proposed protocol, The energy consumption and latency are all reduced well.