Abstract: In order to solve the problem of fast synthesis of frequency variant coupled diplexers, a coupling matrix optimization synthesis method is proposed. The method is divided into two stages which are channel filter synthesis and optimization. Firstly, the characteristic polynomials of each channel filter are obtained iteratively by using the derived diplexer characteristic polynomials, and the normalized coupling matrices of each channel filter are calculated by using the coupling matrix synthesis method. Then, according to the specified coupling topology, the approximation of the canonical coupling matrix is optimized. Finally, the final design parameters are obtained through denormalization process. The simulation results agrees well with the theoretical synthesis results, which shows the correctness and effectiveness of the method. The proposed method shows that the passband of the diplexer maintains better equiripple characteristics and the coupling topology of each channel filter can be arbitrarily specified according to the specifications.