The main characteristics of the third harmonic emission generated by femtosecond laser pulses propagating in air are investigated by numerically solving the coupled nonlinear Schr(o)dinger equations. Strong third harmonic emission is observed with a maximum conversion efficiency as high as 0.43%. The on-axis phase difference between fundamental beam and third harmonic is investigated. The result is in good agreement with the phase-locking mechanism. Dependence of the conversion of third harmonic emission on focusing conditions is also studied. The results are also compared with those of experiments.