The invention provides an S/X/Ka three-frequency feeding source. The S/X/Ka three-frequency feeding source comprises an X/Ka dual-band feeding source horn and an S-band feeding source horn, wherein the S-band feeding source horn is arranged at the periphery of the X/Ka dual-band feeding source horn, radio frequency signals received by four chamfer horns of the S-band feeding source are divided into a left rotation circular polarization signal and a right rotation circular polarization signal by respectively polarizer, a sum signal and a difference signal in left-right rotation circular polarization of an S band are formed by a synthesis network, a radio frequency signal received by the X/Ka dual-band feeding source horn forms a sum radio frequency signal and a difference radio frequency signal in left-right rotation circular polarization of an X band by an X/Ka common waveguide, an X/Ka wave divider, an X tracker, a phase shifter and an X orthogonal device in an X/Ka dual-band feeding line network, the radio frequency signal received by the X/Ka dual-band feeding source horn forms a sum radio frequency signal and a difference radio frequency signal in left-right rotation circular polarization of a Ka band by the X/Ka common waveguide, a Ka tracker, a Ka polarizer and a Ka orthogonal device. By the S/X/Ka three-frequency feeding source, the S/X/Ka three-frequency feeding source can achieve the characteristics of high gain, high polarization identification ratio, low side lobe and the like in the whole working band.