The invention relates to the field of test instruments, and specifically discloses a broadband directional coupler. The directional coupler is composed of multiple coupling sections connected in sequence. Each coupling section includes two coplanar coupling strip lines, an upper dielectric above the coupling strip lines, and a lower dielectric below the coupling strip lines. The two coupling strip lines in the same coupling section are arranged in parallel and are of the same size. In the same coupling section, the upper dielectric and the lower dielectric are of the same size and material. The coupling strip lines in different coupling sections are connected end to end, and are in the same plane. By using dielectrics of different dielectric constants in different coupling sections, the intervals of the coupling strip lines in different sections are approximately equal, and the widths of the coupling strip lines in different sections are approximately equal. Through the design method, the coupling fluctuation and discontinuity of the directional coupler are reduced, and the directionality of the directional coupler is enhanced obviously.