We report the experimental observation of evolving phenomenon of a split multi-wavelength bright-dark pulse in the nonlinear amplifying loop mirror (NALM)-based mode-locked thulium-doped fiber laser (TDFL) with a figure-eight configuration. Bright-dark pulse with 10 wavelengths was successfully obtained at the pump power of 3 W. The time interval between the bright and dark pulses was discovered not only increasing linearly with the pump power but also approximately equaling to the reciprocal of modulation frequency of radio frequency (RF) spectrum. Moreover, we also observed that the spectrum of split multi-wavelength bright-dark pulse can further present up to 13 wavelengths.