Third harmonic generation (THG) experiment of supper intense femtosecond laser with a single BBO crystal of 1.5 mm was carried out. The pulse width of the input fundamental field is about 100 fs, the bandwidth is about 25 nm, the energy is about 6 mJ, and the maximum intensity is 200～900 GW/cm2 after focusing by a lens. The conversion efficiency was 0.8% with the intensity of the input fundamental field of about 300 GW/cm2. Based on the numerical techniques such as split-step Fourier transformation and fourth-order Runge-Kutta method, theoretical analysis was presented and the results show that the third harmonic field is mainly produced by the third-order nonlinear effect; the large bandwidth of the fundamental field is one of the chief reasons that limit the conversion efficiency; the nonlinear phase-mismatching and the conversion efficiency could be compensated well after optimizing the incidence angle and the azimuth angle of the fundamental field.