In the considered multiple access network, the primary users transmit in the allocated mutual orthogonal channels. Meanwhile, the secondary user conducts spectrum sensing and seeks transmission opportunities in the temporarily unoccupied primary channels. In order to improve the stable throughput of secondary user, we propose a cognitive relay strategy, which suggests the secondary user to intelligently relay packets for some selected primary users in addition to sending its own packets. The criterions of selecting primary users are given in system with and without sensing errors, respectively. Numerical results show the benefits of cognitive relay strategy in terms of secondary throughput.