Abstract: For underlay non-orthogonal multiple access (NOMA) cognitive networks using simultaneous wireless information and power transmission (SWIPT), a two-stage relay selection strategy (TSRS) based on NOMA and successive interference cancellation protocols is proposed. The source node and the selected relay of the secondary network both generate multiuser superimposed signals using the same power allocation. The relays use only the harvested energy to provide de-code-and-forward services. The closed-form expressions for users’ outage probabilities are derived and verified by Monte Carlo simulations. The influences of system parameters such as power allocation factor, energy splitting parameter, interfer-ence temperature constraint, and the number of relays on the outage performance is analyzed. Simulation results show that the proposed scheme can significantly improve the outage performance of the system.