Opportunistic spectrum access for a single secondary user under periodic sensing framework is analyzed aiming for the unslotted primary user network. By constructing the secondary user channel sense and access model, an opportunistic spectrum access algorithm based on user request data packet length is proposed. The proposed algorithm could automatically modify the strategy of opportunistic spectrum access according to the service data packet length allocated by each slot. Experiment results show that the proposed algorithm can enhance the secondary users expect effective throughput under the condition of tight interference, and realize the tradeoff between effective throughput and collision probability. It behaves strong robustness against the exotic environment changes.