Abstract: ‍ ‍In this paper， we argue that the high mobility of unmanned aerial vehicles （UAVs） can bring about the dynamic changes of channels， so that the legitimate channel gain can be enhanced and the eavesdropping channel gain can be weakened by controlling UAV position reasonably， thereby improving the secrecy rate between the legitimate nodes. Different from most existing UAV air-ground channel models， in which only the line-of-sight （LOS） transmission link is considered， this paper investigates the physical security scheme under a channel that includes the non-line-of-sight （NLOS） and LOS links. Further we establish UAVs cooperative communication scenarios under the presenc of estimation error of the eavesdropper’s position. The joint optimization of the UAV flight trajectory and the transmit power is studied to achieve the maximum secrecy rate， which is a non-convex optimization problem in the above scenario. To deal with the optimization problem， Successive Convex Approximation （SCA） and Block Coordinate Descent Algorithm （BCD） are used to solve this problem. The simulation results show that with the existence of both line-of-sight and non-line-of-sight links， by effectively planning the UAVs flight trajectory and transmit power， the secrecy rate of UAV communication system can be greatly improved.