Long Term Evolution (LTE) systems will apply on unlicensed spectrum of 5GHz band, to meet explosive growth of traffic volume in the future. In this paper, the power control problem is studied for LTE-U and WiFi coexisting system. We present an adaptive water-filling algorithm to maximize LTE-U throughput while guaranteeing WiFi throughput. The main contribution is to provide a methodology to obtain optimal transmit power and transmission time of LTE-U small cells according to channel state information and WiFi information. Simulation results show the proposed algorithm has better throughput performance and lower complexity.