How to adjust parameters to adapt to the dynamic network conditions is a main problem in wireless communications. There are not any effective methods to deal with this problem, especially in non-saturation conditions. By analyzing feedback signal of channel, we introduce a linear adjustment rules of CW about idle slot intervals of channel, which can be used in non-saturation conditions. And then, we propose a fully adaptive access mechanism, which adjusts parameters dynamically according to the traffic loads in the network. With the linear adjustment of CW and measurement of idle slot intervals, the mechanism separates the adjustment rule of parameters from the status of data transmissions, which is distinguished from that in the DCF method. By sensing the channel idle slot intervals, this mechanism does not need to know or judge whether in saturation conditions or not, which simplify the process of access mechanism. The simulation results demonstrate the validity and good scalability of the proposed access mechanism.