The invention discloses a broadband confocal waveguide HE0n mode excitation device, belonging to the technical field of vacuum electronics. The device comprises an output open confocal waveguide section, a closed confocal waveguide mode transition section, a cutoff transition section and an even open waveguide section. A Y-shaped 3dB power divider is arranged at the junction of the closed confocal waveguide mode transition section and the cutoff transition section. The two-channel output waveguide is bent inwardly by 90 degrees and perpendicular to the input waveguide; and the output port receives two microwave signals of equal amplitude in opposite directions. The device of the invention adopts a doubly-fed structure which is perpendicular to the confocal waveguide mirror surface, and hroguh the two pairs of reflection mirrors of the cutoff transition section, high-efficiency conversion of the confocal waveguide HE0n mode can be realized in the broadband range with the flatness in the band. In the millimeter wave and terahertz band, this can be realized by traditional processing methods; for the low sensitivity structural parameters, the device can be directly applied to the input coupling device of a millimeter wave and terahertz band confocal waveguide convolution amplifier and the cold tests for high frequency system characteristics of the convolution amplifier.