Abstract: Aimed at efficient and fast searching of spectrum opportunities, while avoiding interference to primary users, spectrum sensing play a significant role in cognitive radio system. Generally, multi-user cooperative sensing can enhance the performance of system. Hence, this paper is focus on the characteristics of two kinds of sensing mechanism such as proactive and reactive sensing policy in a multi-user cooperation situation, and analyzes the sensing performance over the activity level of primary users. We proposed a optimization strategy of making a tradeoff between access delay and sensing overhead. Through the theory analysis and simulation, multi-user cooperative sensing in proactive and reactive mechanisms also reduce the channel searching interval.