In cognitive radio networks channel reservation has been a branch of deploying more efficient way to the utilization of spectrum. Because of the dominating status of primary users, secondary users have to make room for them when they come. Channel reservation is defined to reserve several fixed channels only for primary users, which will lead to less frequency spectrum handoffs comparing with no reservation mechanism. However, it is a waste of secondary traffic when the reserved channels are not all occupied. In this paper we define a new way to control the number of reserved channels with flexibility so as to enable more secondary user traffic to serve in the cognitive radio networks. We define system maximum tolerable conditions as Grade of Service to judge the performance. Simulation results show that under the same circumstances of the system maximum tolerable probabilities, dynamic channel reservation mechanism works better than fixed channel reservation mechanism by an increase of about 26% traffic gain.