Abstract: Certain gyro-type devices, including moderately high power, high harmonic gyrotrons, gyro-TWT and peniotrons, require an axis encircling electron beam. To develop practical and efficient devices of this type, it is necessary to first develop a gun that generates a high quality beam with low velocity spread and ripple. This paper analyzes the large orbit gyrotron cusp electron beam, builds up the physical models of the cusp electron beam, and discusses the effect on the factor in designing the large orbit gyrotron cusp electron gun based on theoretic analysis. The cusp gun designed can generate an axis encircling beam with low-velocity spread and small beam ripple for high-harmonic high-power (52 kV, 2.7 A) operation in gyrotrons.