In this paper, we present a dual-hop transmission system composed of underlay cognitive radio frequency(RF)network cascaded with multiple input multiple output (MIMO) free-space optical communications (FSO) links, and analyze the outage probability. For the RF link, we use the Rayleigh fading distribution model and analyze an underlay cognitive radio network where the secondary users share the spectrum with licensed primary users. For the FSO link, we use the unified Gamma-Gamma turbulence distribution model to establish the MIMO FSO channel model derived with the equal gained diversity combined technique considering the effects of atmospheric attenuation and turbulence. We derive a new exact closed-form expression for the outage probability under fixed amplify-and-forward relay scheme. The influences of various weather conditions and turbulence strength on the cascade link are verified via computer simulations. The results indicate that the performance of the mixed underlay cognitive radio network and MIMO FSO system is increased and the atmospheric effect is decreased gradually with the increase of the peak transmission power of secondary user transmitters and the number of transmitting aperture and receiving aperture.