In this paper, we present a special spectrum sharing scheme that is a joint optimization of relay selection and power allocation at the secondary transmitter, where the primary user is incapable of supporting its target signal-to-noise ratio(SNR). Specifically, the selected secondary transmitter assists the primary user with achieving its target SNR via two-phase cooperative amplify-and-forward relaying. By searching for the candidate secondary transmitters which have already satisfied the primary user’s target SNR, we can select the optimal secondary transmitter. This optimal secondary transmitter not only satisfies the primary user’s target SNR,but also maximizes the throughput of the secondary user. We study this joint optimization problem such that the secondary user’s throughput is maximized under the constraint that satisfies the primary user’s target SNR.Numerical results show that our scheme can maximize the throughput of the secondary user, and can obtain the win-win solution for the primary and secondary systems. In this paper, we present a special spectrum sharing scheme that is a joint optimization of relay selection and power allocation at the secondary transmitter, where the primary user is incapable of supporting its target signal-to-noise ratio (SNR). selected secondary transmitter assists the primary user with achieving its target SNR via two-phase cooperative amplify-and-forward relaying. optimal secondary transmitter not only satisfies the primary user’s target SNR, but also maximizes the throughput of the secondary user. We study this joint optimization problem such that that the secondary user’s throughput is maximized under the constraint that satisfies the primary user’s target SNR. that our scheme can maximize the throughput of the secondary user, and can obtain the win-win solution for the primary and secondary systems.