To investigate the intrinsic mechanism of femtosecond laser filament formation to achieve length characterization, fluorescence spectra and acoustic signal information of the plasma filament at different incident laser pulse energies and polarization states were obtained by fluorescence and acoustic methods. The results show that, the intensity of N2 fluorescence signal produced by circularly polarized light is about twice of that of linearly polarized light with the same laser pulse energy, while the intensity of N2+ fluorescence signal produced by linearly polarized light is about 1.3 times of that of circularly polarized light; Plasma fluorescence measurement is an effective way to obtain the length of plasma filament, which is more accurate than acoustic measurement. This study provides a feasible solution to characterize the filament length by revealing the physical essence of coherent laser emission with filament length.