Abstract: Some structural and electric parameter characteristics of the MBK were explained in the article. Approximate formula on the output cavity coupling coefficient of the MBK was driven, using method of the normal mode analysis for a pillbox output cavity. Bandwidth characteristics of the MBK were analyzed preliminary in this paper. It concluded that the increase of product of MBK’s output efficiency and its bandwidth completely depends on the increase of beam’s perveances. It also concluded that the output cavity coupling coefficient and the characteristic impedance of the cavity decrease as the beam number grows, but they decrease more slowly than the increase of perveances as the beam number grows.