The invention discloses a strip line directional coupler and a coupling degree adjustment method thereof, wherein the directional coupler comprises: a first ground layer, a strip line layer and a second ground layer that are orderly stacked; the strip line layer includes a first line and a second line, the first line and the second line are provided with coupling sections that are close to each other, and the first ground layer and/or the second ground layer are provided with adjustment zones that are facing or close to the coupling sections. The adjustment zones are provided with openings forcoupling degree adjustment. The adjustment zones of the ground layers are provided with the openings for coupling degree adjustment. Via change of the shape, area and quantity of the openings, current distribution of a strip line is changed, and simple and effective coupling degree adjustment can be realized.