A novel full-duplex (FD) multi-threshold cooperative sensing scheme based on phase difference (PD) is presented in this paper. Compared with single-threshold cooperative method, the novel multi-threshold cooperative method proposed in this paper can greatly improve the performance of the FD cognitive radio networks (CRNs). PD method is used in this paper since it is immune to the signal-to-noise ratio (SNR) level. Besides, two different kinds of weighing methods are presented in this paper, i.e., weighing based on the average of every secondary user's (SU) report and weighing based on Bias discriminant. Closed-form solutions for both non-cooperative case and weighing based on the average of SU's report are proposed in this paper. Simulation results justify that our closed-form solutions are correct, and our multi-threshold weighing cooperative method can improve the performance of the FD system.