Abstract: Wireless sensor network generally runs in the dynamic physical environment which outside environment is constantly changing, once some sudden incident happens, sensor network task load will be caused changeable greatly which will affect the network system performance seriously. Therefore, it is very necessary to design a new MAC protocol for network load sudden change. In this study, based on analyzing the advantages and disadvantages of S-MAC protocol and existing methods, we put forward a new improved S-MAC protocol for network load changeable sharply which chooses different methods for different nodes to realize the equity of energy and time delay.