Owing to retransmission of lost packets and bursty nature of source data, even an ad hoc network with low average traffic may suffer heavy traffic in some certain periods, which in further may degrade system performance, particularly in multi-hop wireless communication. Aiming to provide a solution to above problem, in this paper, we construct a hybrid system based on plural ad hoc networks, which are defined as primary radio networks (PRNs), and a secondary radio network (SRN). In the hybrid system, a cooperative relay scheme based on dynamic spectrum access, defined as cognitive relay, is adopted for retransmitting the lost packets of PRNs. Cognitive relay is carried out by SRN in spectrum hole. Simulation-based evaluation shows that the proposed cognitive relay, compared to existing retransmission protocol, is more effective in providing QoS-guaranteed communication.