In this paper, by investigating the terahertz(THz) waveforms emitted from different lengths of filaments via THz time-domain spectroscopy system(THz-TDS), superluminal propagation of THz wave during filamentation was observed. It implies that the refractive index of the THz wave is smaller than the unity within the filament region, and thus the THz pulse may propagate inside the filament. This hypothesis is supported by further numerical simulation, which demonstrates THz eigenmode inside the plasma filament.