The number of competing stations has great influence on the performance of IEEE 802.11 MAC protocol based on the distributed coordination function (DCF), which utilizes carrier sense multiple access with collision avoidance (CSMA/CA). In this research, it is found that the system performance approaches the optimal values with the same protocol parameters, when the number of competing stations dynamically changes within a certain range. Therefore, an adaptive optimization mechanism, DOOR (dynamic optimization on range), is proposed for the IEEE 802.11 DCF, which is based on the condition detection and range of competing station number. Moreover, the principle and method for partitioning the range of competing station number are also introduced. Later on, the detailed system model and performance evaluation for the new mechanism are given. The elaborate numerical results show that the mechanism could achieve much higher throughput and shorter delay than the standard IEEE 802.11 DCF in almost all the different competing stations numbers.