Abstract: In this paper, we study an underlay cognitive wireless powered communication network (CWPCN). In this model, there are a primary transmitter and two secondary user pairs, each of which contains a secondary transmitter and a secondary receiver. The secondary transmitter can work in both backscatter communication (BackCom) protocol and harvest-then-transmit (HTT) protocol. To maximize the system throughput of the secondary network, we investigate three cases depending on whether the harvested energy can meet the circuit power consumption of secondary users. For each case, the optimal time allocation policy is derived. Simulation results show that the performance for the proposed model in this paper outperforms that of the traditional models and cooperation CWPCN.