In this paper, a 5-pole reconfigurable and tunable diplexer with high isolation based on a pseudo-combline structure is designed and simulated. The proposed diplexer has two BPFs for both channels that operate at specific bands S (2 GHz) and L (3.5 GHz). A new class of feed section is proposed to increase the matching level between BPFs to make them operate at the desired frequencies. Varactor diodes are used to obtain the tunability property for the proposed diplexer, where they can tune the two bands of the diplexer to different frequencies. Also, for reconfigurability, a pin diode can be used as a switch in the feed section between to BPFs. The proposed diplexer has a compact size (46.12*40.49) mm; the insertion loss of two channels is 0.86, 0.39 dB; and the return loss is 12.24, 18.47 dB. Finally, the proposed diplexer has the highest isolation between TX and RX ports (more than 69 dB), which is higher than other related works.