A multi-channel mode converter operating with a series of TE or TM mode electromagnetic wave includes a plurality of coaxial waveguides arranged in overlay configuration. By controlling radius ratio and the number of coupling aperture of each coaxial waveguide, high power and high purity of operating mode of electromagnetic wave can be obtained and the major parasitic mode of electromagnetic wave can be suppressed, so as to avoid crosstalk between coaxial waveguides. A rotary joint including the above-mentioned mode converter with multi-channel is also disclosed.