A third harmonic generation method using dual non-collinear laser filaments of equal wavelength was applied to experimentally study the spectral properties of the on-axis third harmonic generation in nitrogen gas. The filaments were generated by non-collinear focusing two beams with 820 nm of central wavelength,70 fs of duration,10 Hz of repetition,3 mJ and 7 mJ of single pulse energy into a nitrogen gas cell,using a concave mirror. When the two laser beams were overlapped both in time and space,the intensity,central wavelength and spectral width of the harmonic signal were found to be strongly dependent on the gas pressure and time delay of the two fundamental beams. The enhancement,spectral broadening/narrowing of the third harmonic spectra were observed when changing the time delay of the two fundamental beams. When the pressure of nitrogen gas about 0.16 MPa,and the inducing beam advanced the stimulating beam of about 20 fs,the on-axis third harmonic signal on the stimulating beam side was enhanced of about 60 times,while its spectral width was broadened from 15 nm to 25.8 nm,and 100 THz in frequency domain.