Abstract: As the threat from criminals to public safety is increasing, proactive eavesdropping begins to play an important role in the field of commercial and military. In view of this suspicious communication with decode-and-forward relay network, a proactive eavesdropping system of hybrid duplex is proposed. Assuming that when the suspicious receiver is disturbed, the transmission rate of the suspicious sender is forced to change in order to maintain a constant outage probability. Take advantage of this, the interference terminal of proactive eavesdropping is optimized to maximize the average eavesdropping rates. And besides, the average eavesdropping rate and the optimal jamming power under two different signal combination schemes at the eavesdropping receiver are analyzed. Finally, the eavesdropping performance of the system under the optimal jamming strategy is compared with that of other systems in the reference. The expression and simulation results show that, under the same optimal jamming strategy, the hybrid duplex proactive eavesdropping system proposed in this paper has the best proactive eavesdropping performance when the receiver adopts the maximum-ratio combined scheme.