Abstract: The characteristics of spectral distribution of harmonic emission are analyzed with a single electron model in the cases of circularly polarized femtosecond laser pulses. It is discovered that the oscillation of electrons will generate harmonic emission due to their relativistic motion,and broadening and redshift in the spectrum of harmonic emission occur with the increase of laser intensity. The effects discussed in this paper can explain qualitatively the broadening and redshift of harmonic emission from plasma irradiated by femtosecond intense laser pulses．