Abstract: The non-orthogonal multiple access (NOMA) technology was applied to cognitive networks, and an underlay cognitive NOMA system model based on relay cooperation was proposed. Under the Rayleigh fading channel, considering a two-hop communication network, the cognitive relay could receive signals from the primary network transmitter and the secondary network transmitter, and applied the successive interference cancellation technology to decode-and-forward the signals it received. The secondary network got the opportunity to access the authority spectrum through providing decode-and-forward service for primary network. The primary network obtained the opportunity for secondary network cooperative communication by providing the authorized frequency band for the secondary network, similarly it got the priority decoding right, which ensuring the reliability of the communication. Closed expression of the outage probability was derived and simulated by MATLAB. The simulation results show that relay cooperation can effectively improve the outage probability performance.