A tunable narrowband THz filter with a sub-wavelength array structure based on FDTD simulation is presented. Since the array structure is adjustable in the size range of 16μm to 160μm, its filtering frequency can be tunable in a wide range of 2.5THz to 20THz with a minimum bandwidth down to 0.04THz. With this filter, excellent narrowband filtering is achieved. In addition, the structure can change the THz wave polarization by changing the angle between itself and the THz wave. So it can achieve both narrowband filtering and polarization. Compared with the traditional structures, this structure has the features of simplicity and easy operation. It can improve the resolution of THz imaging systems and is of significance to the future research on THz imaging.