Past a few years have witnessed the explosive development of unmanned aerial vehicle (UAV)- assisted wireless networks, owing to the fast and flexible deployment in emergency scenarios, such as temporary hot-spot, nature disasters, and so on. However, due to limited batteries carried by UAVs, energy consumption is one of the most fundamental concerns in UAV-assisted wireless networks. In this paper, we propose an energy-efficient resource allocation scheme which considers employing wireless power transfer (SWIPT) technology to power UAVs. Particularly, we jointly optimize the power splitting ratio, the UAV's position, and the time slot division ratio. The problem is decoupled into two sub-problems and solved iteratively. Simulation results show that the proposed scheme can reduce the UAV's energy consumption significantly.