The invention discloses an S/X double-band circularly polarization feed source which adopts a coaxial cavity structure and comprises an S frequency band horn, a caliber transforming section, an S frequency band coupling hub, an S frequency band circularly polarization forming network, an X frequency band horn, an X frequency band circular polarizer, a cable component and a waveguide component. The outer cavity of a coaxial cavity comprises the S frequency band horn, the caliber transforming section and the outer cavity of the S frequency band coupling hub, and the inner cavity of the coaxial cavity comprises the X frequency band horn and the inner cavity of the S frequency band coupling hub. The S/X double-band circularly polarization feed source adopts a coaxial feeding source design scheme, the coaxial cavity realizes the radicalization of S frequency band circularly polarization electromagnetic wave, and the X frequency band multimode horn embedded in an inner core of the coaxial cavity structure to be as the feed source realizes the radicalization of X frequency band circularly polarization electromagnetic wave; and according to the design thought of the high-gain S/X double-band circularly polarization feed source, by performing special design aiming at high gain on the S frequency band coupling hub, and smartly combining X feed source design to perform the integrated design of the feed source, the S/X double-band circularly polarization feed source becomes a novel high-gain S/X double-band circularly polarization feed source which covers deep space detection, observation and control communication application frequency bands, and has good radiation property.