Abstract: In view of the scarcity of wireless spectrum resources and the security problems of air to ground line of sight （LoS） links， a secure transmission scheme of cognitive radio network （CRN） assisted by unmanned aerial vehicle （UAV） relay is proposed， UAV acts as a decode and forward mobile relay to assist the secret transmission from the source node to the legitimate destination node. The purpose is to maximize the security rate by optimizing the flight path and transmit power of UAV relay under the power constraints of source node and relay node. Since the design problem is nonconvex， a surrogate function with nonconvex constraints is constructed to approximate the original problem to convex constraints， and an iterative algorithm based on successive convex approximation is used to solve the problem. The simulation results show that compared with the static relay and no-optimization schemes， the proposed joint optimization scheme not only obtains the optimal path of UAV flight， but also achieves the average security rate of 1.06 times and 2.88 times of the traditional schemes， respectively.