Radio-frequency excitation enables the plasma to contain two, three or more harmonic waves, and the modulating signal produced by signal source and excitation source under the plasma nonlinear influence should not be ignored. A new wave-plasma interaction model is set up considering the electron thermal motion, recombination and diffusion effect. It verifies the modulations of signal in different frequencies and explains the production of the second and high order harmonic waves in plasma excitation. The variation of the EM wave pedigree is analyzed taking into account the parameters such as input power, ionization field threshold and electron average energy. The result is of use in tuning the matching circuit or selecting the coupling element in plasma discharge, and it is helpful in electric magnetic interface design, plasma antenna transmission, etc.