According to the analytical model that is used to study the performance evaluation of the IEEE 802.11 Distributed Coordination Function (DCF), we analyze how the size of the initial contention window works on the saturation throughput and the energy consumption. We also provide a new integrate metric to measure both the throughput and the energy consumption. Considering the situation of the constant station number, we preset a optimal CW_min to achieve the maximum performance. On the other hand, we propose a new algorithm called Self Adjusting CW_min(SACW) for the situation of the time-varying station number. The paper gives the simulation results by means of the new methods and compares them with the original 802.11 protocol.