The invention discloses a multipath Ka-waveband radial waveguide power distribution/synthesis network and a method and application thereof. An input/output step-shaped waveguide, a circular waveguide, a multipath radial waveguide cavity and a coaxial probe are included, the multipath radial waveguide cavity is connected with the input/output step-shaped waveguide, the coaxial probe is inserted into the middle portion of a radial waveguide, and extends into the input/output step-shaped and rectangular waveguide after multiple times of gradual change, matching of a coaxial waveguide to the step-shaped waveguide and matching of the coaxial waveguide to the radial waveguide are formed, and two power distribution/synthesis networks can form a complete power amplifier via a back-to-back structure. The rectangular waveguide serves as an input/output port and can be connected with other millimeter wave devices, and the gradual-change probe is used and the step-shaped structure is added to the input/output rectangular waveguide to expand the bandwidth of a radial waveguide power distribution/synthesis device.