A branch-line coupler includes an input port, arranged on a bottom layer of the substrate; an isolated port, arranged on the bottom layer; a first output port, arranged on a top layer of the substrate; a second output port, arranged on the top layer; a ground layer, arranged in a middle layer of the substrate; a first transmission line, connected between the input port and the isolation port; a second transmission line, connected between the first output port and the second output port; a first branch line, connected to the input port; a second branch line, connected to the isolated port; a third branch line, vertically connected to the first output port and connected to the first branch line through a first through hole; and a fourth branch line, vertically connected to the second output port and connected to the second branch line through a second through hole.