A novel THz fiber Bragg grating based on a polymer THz fiber filter was proposed and investigated The periodic index modulation of the THz fiber Bragg grating was introduced by the periodic diameter modulation of the polymer THz fiber by using a carbon dioxide laser or an ultraviolet laser to process the polymer THz fiber point by point. Based on the finite element method and fiber Bragg grating theory, the properties of the proposed THz fiber Bragg grating are well investigated considering the peak wavelength, reflectivity, bandwidth, length of the THz fiber Bragg grating and also the diameter and the degree of diameter deformation of the polymer THz fiber. The simulation results show that nonlinear relationship between the peak wavelength (reflectivity) and the grating pitch of the THz fiber Bragg grating is quite different from the conventional optical fiber Bragg grating.