Based on the nonlinear coupling equations, conversion properties (i.e. efficiency and bandwidth) of third-harmonic generation (THG) of ultrashort pulses in different special group-velocity modes are analyzed. The THG mixing process of ultrashort pulses (FWHM～150 fs) in Type-I KDP crystals is simulated by using split-step Fourier transformation and fourth-order Runge-Kutta method. Obtained results show that THG conversion properties (conversion efficiency being 50% and frequency bandwidth up to 2 nm) in the “three-wave-packet unequal velocity-mode” are better than those in other group-velocity modes. The conclusion provides theoretical guiding for further improving the THG efficiency and bandwidth of ultrashort pulses.