In this paper, we study a wireless powered communication network (WPCN) subject to the co-channel interference. In the down-link (DL), the hybrid access point (H-AP) broadcasts the wireless energy offered by the radio frequency (RF) signal to the user and relay. In the up-link (UL), the user and relay nodes transmit information to the H-AP through time-division-multiple-access (TDMA). In contrast to conventional WPCN, the interference can be considered as additional energy, not just as a threat for the data transmissions. Through the manner of curve fitting, we obtain the closed-form expression for the system outage probability and the throughput. Furthermore, the optimal time allocation to maximize the average system throughput is obtained. Finally it is shown that the theoretical results fit the simulation results closely and the proposed optimal scheme achieves the best performances..