Using a theoretical model for generation of the third_order harmonic in a gas_fi lled capillary driven by fs laser pulses, the signal_pressure curves of the thir d_order harmonic generation in a gas_filled capillary driven by fs laser pulses are discussed. It is found that when the parameters of the capillary and laser pulse are given, the generation efficiency of the third harmonic is influenced b y the waveguide modes of the third harmonic, the pressure, the walk_off and the phase modulation, and that the theoretical value of the generation efficiency of the third harmonic, which is given in this paper, approaches the experimental one.