Low-frequency noise in the hydrogenated amorphous silicon thin film transistor is investigated in this paper. The drain current noise spectral density shows a 1/fγ (γ ≈ 0.92, f represents frequency) behavior which ascribes to fluctuations of the interfacial trapped charges due to the dynamic trapping and de-trapping of free carriers into slow oxide traps and localized traps. The normalized noise has the power law dependence on overdrive voltage, and the power law coefficient is about -1 which illustrates that the flicker noise is dominated by mobility fluctuation mechanism. By considering the contact resistance, and emission and trapping processes of carriers between localized states in the Si/SiNx interface, the variation of low frequency noise with drain current is analyzed and fitted by use of the theory of carrier number fluctuation with correlated mobility fluctuation (ΔN-Δμ model). Furthermore, the relationship between surface band-bending and gate voltage is extracted based on subthreshold current-voltage characteristics, and thus the density of localized states is then extracted through the measurement of drain current noise power spectral density. The experimental results show an exponential localized state distribution in the band-gap while densities of two defect modes at the bottom of conduction band NT1 and NT2 are about 6.31×1018 and 1.26×1018 cm-3·eV-1, and corresponding characteristic temperatures TT1 and TT2 are about 192 and 290 K, which is similar to the reported distribution of tail states in the amorphous silicon layer. Finally, the average Hooge's parameter is extracted to estimate the quality of devices and materials.