This paper proposes an energy-efficient medium access control (MAC) protocol based on posture-aware approach for wireless body area network (WBAN) under walking scenery. Person's movements could bring about both the network topology and relative positions of sensor nodes changing, which means that the protocols designed under the static environments are no more suitable for dynamic WBAN. For this question, firstly, we analyze the feature of people walking at a constant speed and divide a spell of movements into a sequence of key frames just like a film constituted by numbers of continuous frames. As a result, the dynamic walking process is translated into several static postures, which the static MAC protocol could be used for. Secondly, concerning the performance of network lifetime, we design a posture-aware dynamic protocol for lifetime maximization (PA-DPLM). With analytical and simulation results provided, we demonstrate that PA-DPLM protocol is energy-efficient and can be used under walking scenery.