Third harmonic generation (THG)of super intense femtosecond laser pulse with one BBO crystal was analyzed. Effects of the third-order and the cascaded second-order nonlinearity in one crystal on the third harmonic conversion efficiency were compared. Influences of the factors, such as input fundamental intensity, length of the crystal, self-phase-modulation (SPM) and cross-phase-modulation (XPM), group-velocity mismatching, detuned angle, azimuth angle, etc, on the conversion efficiency, pulse shape and spectra of the third harmonic field were discussed in detail. Furthermore, the methods to improve the third harmonic conversion efficiency were proposed. The results show that, the variation of the maximum intensity and the pulse width (full width at half maximum (FWHM)) of the third harmonic field with the length of the crystal is not obvious. By justifying the phase-mismatching angle of the fundamental field, the conversion efficiency and the maximum intensity of the third harmonic field can be increased, and the pulse width (FWHM) can be reduced. Moreover, it is also helpful to improvement of the conversion efficiency by optimizing the azimuth angle of the crystal.