For diplexer with high isolation and wideband in wireless communication system, in this paper, the diplexer specification is divided into Tx and Rx filter specifications with a certain margin. Then the wideband Tx and Rx filter is designed by using the microstrip interdigital structure and the improved method based on coupling coefficient K and external quality factor Qe. Finally, two kinds of T-junction and filter combination structures are used to further design the diplexer, and the microstrip diplexer with these two structures are simulated and analyzed respectively. The simulation results show that the structure of parallel connection diplexer is compact, but the passband isolation is not satisfactory. The minimum return loss in the passband of the series connection diplexer is 14.16 dB, the maximum insertion loss is 1.01 dB, the minimum passband isolation is 53.46 dB, and the size of the diplexer is 8.089 9 mm×2.059 1 mm×0.302 mm. It has the advantages of large relative bandwidth, high isolation and low insertion loss, which provides the feasibility for the design of high-performance diplexer.