The most energy-wasting part of the MAC protocol for wireless sensor network is at the idling condition, therefore it is crucial for power saving to be able to turn off the signal transducer of the wireless network when the equipment is idling. PMAC allows sensors that did not transfer for a long period of time to quickly enter a dormant state, but this type of design requires precise time synchronization mechanism. To achieve time synchronization is a very energy-consuming and very expensive mechanism in the sensor network structure. Additionally, the exchange action with the neighboring pattern after each cycle, not only generates additional energy consumption for data transfer, but also accompanied by factors such as competition, collision and pattern exchange failure. We propose an Asynchronous MAC protocol (AMAC) in this paper and expect to improve the problem of energy wasting and time synchronization due to sleeping schedule exchange under the PMAC basic protocol.