To deal with the secrecy issues and energy efficiency issues in the unmanned aerial vehicles（UAVs） assisted communication systems, an UAV-enabled multi-hop mobile relay system is studied in an urban environment. Multiple rotary-wing UAVs with energy budget considerations are employed as relays to forward confidential information between two ground nodes in the presence of multiple passive eavesdroppers. The system secrecy energy efficiency（SEE）, defined by the ratio of minimum achievable secrecy rate（SR） to total propulsion energy consumption（PEC）, is maximized via jointly optimizing the trajectory and transmit power of each UAV relay. To solve the formulated non-convex fractional optimization problem subject to mobility, transmit power and information-causality constraints, an effective iterative algorithm is proposed by applying the updated-rate-assisted block coordinate decent method, successive convex approximation（SCA） technique and Dinkelbach method. Simulation results demonstrate the effectiveness of the proposed joint trajectory design and power control scheme.