Cognitive radio and full duplex are regarded as two promising techniques to enhance the spectrum utilization efficiency. This paper studied the bi-directional communication underlay full duplex cognitive radio networks, which combines these two means together. Considering the instantaneous channel state information of the primary user (PU), a joint power allocation scheme between single input single output secondary users (SUs) is designed to enhance the throughput of the second transmission, while satisfy the PU's quality of service requirement. Finally, we show the superiority of the proposed scheme via simulation results.