Strong forward propagated THz pulse can be generated by femtosecond laser filamentation in air. An electro-optical sampling based time domain spectroscopy (TDS) technique is used to diagnose the THz pulse produced by a femtosecond laser filament in air. The filament is generated by a 1 mJ, 50 fs laser pulse centered at 800 nm. The THz pulse measured in experiment is a single-cycle pulse (pulse width is 1ps) and the corresponding spectrum peak value is around 1 THz. The polarization orieatation of pump beam is rotated and the variation of electro-optical sampling signal is analyzed-Compared the experimental results with the calculational results, the polarization of THz pulse emitted from filamentation is elliptical.