Aiming at the problems of energy limitation and secure communication of Unmanned Aerial Vehicle （UAV） communication， by jointly optimizing the transmitting power and flight trajectory of UAV， and aiming at the maximization of average secrecy capacity of the system， a security scheme of mobile edge computing system based on UAV Wireless Power Transfer （WPT） was proposed. In this scheme， due to the non-smooth nature of the objective function， it is difficult to solve directly. To solve the above problems， firstly， the non-smooth objective function was reconstructed into an equivalent smooth objective function. Then， the reconstructed objective function was decoupled into two sub-problems， namely， optimizing the transmitting power for a given flight trajectory and optimizing the flight trajectory for a given transmitting power. And the optimal solutions of the two sub-problems were solved using the two-dimensional binary search method and the successive convex approximation algorithm respectively. Finally， the suboptimal solution of the problem was found through the block coordinate descent method， and the two sub-problems were solved iteratively and alternately. The results of comparing the proposed scheme with the trajectory optimization scheme without transmitting power control， the optimal trajectory optimization scheme with transmitting power control， and the optimal trajectory optimization scheme without transmitting power control show that： as the flight time increases， the average secrecy rate of the proposed scheme is increased by 36.0%， 9.2%， and 34.8%， respectively； as the transmitting power increases， the average secrecy rate of the proposed scheme is increased by 12.4%， 3.0%， and 14.4%， respectively. So the proposed scheme can effectively improve the average secrecy rate of the system.