Abstract: In order to solve the problem that the traditional directional coupler has a narrow bandwidth and is greatly influenced by harmonic signals, this paper presents a broadband directional coupler with the function of harmonic suppression. The directional coupler adds a branch section to double-branch-line directional coupler, thus can effectively improve the bandwidth. While its transmission lines are added with low-pass filter structures, it can filter harmonics well. The experimental results show that the directional coupler reflection coefficient and isolation are less than -15 dB, the transmission loss is more than -3.8 dB, the fluctuation range is 0.5 dB and the phase difference is 90°. It satisfies the design requirement of coupler, and can be applied to microwave components to improve the performance.