A mode-selective interactive structure for gyrotrons includes a plurality of metal tubes, wherein an inner wall of each metal tube forms a waveguide; and between each adjacent pair of the metal tubes exists a slice with a first interface and a second interface and when an electromagnetic wave comprising an operating mode and a competing mode propagates through the slice, the competing mode is partially reflected upon, partially transmitted through and/or absorbed at the first interface and the second interface of the slice so that the power loss of the competing mode is larger than the operating mode and the production of the competing modes is suppressed progressively thereby achieving mode selection.