Abstract: Relay cooperative can not only improve the transmission reliability, but also has great potential to enhance the physical layer security for cognitive users. In this paper, we consider a cooperative spectrum-sharing underlay cognitive radio networks in the presence of one eavesdropper, where the transmission power of the secondary transmitter is constrained by the maximum transmission power and the peak interference power to the primary receiver, and both the relays and the secondary destination are interfered by the primary user’s co-channel interference. We design a selection decode-and-forward and distributed zero-forcing beamforming (SDF-DZFB) scheme for physical-layer security Transmission and investigate the statistical characteristics of the received signal to interference noise ratio (SINR) of the links from the secondary transmitter to the secondary receiver and from the secondary transmitter to the eavesdropper, respectively. Then, we derive the closed-form expression for secrecy outage probability over independent and identically distributed Rayleigh fading channels. Furthermore, to reveal insights into the impact on secrecy diversity order, the asymptotic secrecy outage probability is provided. Finally, simulation results are provided to validate our theoretical analysis.