Abstract: Medium access control (MAC) protocol is the basic protocol and technique in wireless sensor networks (WSN) .So an energy efficiency MAC protocol is the base of an energy efficiency wireless sensor network. Energy limited is the main characteristicness of WSNs. By refering to the classcal energy efficiency MACs, an improved scheme is putted forward. Using methods of dynamically adjust the Duty Cycle and avoid the boundary node, the new scheme can increase the energy efficiency of WSN.