Unmanned aerial vehicles （UAVs）assisted simultaneous wireless information and power transfer （SWIPT） can be used to improve energy efficiency of Internet of Things （IoT）. It can replenish energy for ground devices in IoT to complete the task of information-receiving uninterruptedly. In the face of the limited energy of UAVs and the demand for improving energy efficiency， this paper studies an energy-efficient UAV-ground communication optimization problem. We jointly optimize UAV transmit power and power splitting ratio， and design UAV trajectory and ground devices wake-up time allocation， in which UAV propulsion energy consumption and energy demand of ground devices are considered comprehensively. The formulated energy-efficiency maximization problem is a non-convex， fractional and mixed integer programming problem. To solve this problem， this paper proposes an alternate iterative optimization algorithm based on the successive convex approximation （SCA） and the classical Dinkelbach method. Finally， simulation results verify the effectiveness and superiority of the proposed algorithm.