This paper theoretically studies the spectral characteristics of the third harmonic generated (THG) in air by focusing a two-color-field laser, which is synthesized by a femtosecond laser pulse (ω0) and its second harmonic (2ω0). It is found that the third harmonic intensity varies as a function of the temporal delay between the fundamental and the second harmonic wave, showing a pronounced periodic modulation. This observation is explained by the interference taking place between two third harmonic sources. One is induced by the direct third harmonic generation of the fundamental wave, while the other is given rise by the four-wave mixing (FWM) of the fundamental and the second harmonic waves. In addition, the third harmonic spectra are investigated when the chirp and the self-phase modulation (SPM) of the fundamental pulse and the second harmonic are taken into account. Our results can provide some key informations for the coherent control of the third harmonic generation induced by two-color-field femtosecond laser in air.