In recent years, a variety of rapid development of wireless communication has network resulted in the growing shortage of spectrum resources. Most of the wireless spectrum resources are allocated to different independent fixed-type applications. However, some literature shows that the allocated spectrums are low utilization. Therefore, Cognitive Radio was proposed which contains the concept of spectrum reuse. In CR network, the primary user has the higher priority to access the communication channels. Therefore the transmission of the cognitive users must not cause the interferences to the PUs. Consequently, spectrum sensing is the most critical function for realizing the idea of cognitive network. In order to improve the efficiency of spectrum utilization, we focus on cluster based CSS. And we consider multi-user channel access mechanism, which is to take both the sensing stage into considerations to improve the overall system throughput.