With the development of 5G communication technology, the evolution of communication frequency bands has higher and higher requirements on the performance specifications, package size, and integration of acoustic filters. However, the isolation index is a difficulty point in the design of small-sized surface acoustic wave (SAW) duplexers. This paper studies how to improve the isolation of the duplexer. Based on the Coupling-of-Mode (COM) and the electromagnetic simulation model, a simulation model of the duplexer is established. By improving the topology structure, the common ground inductance and package, etc., finally, the design of a Band 5 duplexer with -60.5 dB of isolation at the Tx end and -50 dB of isolation at the Rx end, small insertion loss has been realized. Comparing with the unimproved duplexer, the isolation of the proposed one has been significantly improved.