The present paper utilizes the Green function method to analyze the scattering properties of the metal wire grating in the THz band, and further to study the influence of the diffraction on the transfer characteristic of the metal wire grating. Results prove that the transmission amplitude of the incident electromagnetic wave is increasing monotonously with the enlargement of the value of a/, and it arrives at the maximum in the zero-order diffraction limit. Due to the strengthened effect of diffraction, transmission curves of the incident electromagnetic wave show oscillation in the critical frequency transition region. Due to the diffraction effect, the transmission amplitude of the incident electromagnetic wave demonstrates an overall decline in the diffraction zone, and it is decreasing monotonously with the enlargement in the value of a/. Compared with microwave transmission line method, this method can aliminate the restriction that the wavelength of the incident electromagnetic wave must be greater than the wire grating constant, and the thickness of wire grating must be far less than the metal line width. This method could be used to analyze the distribution of the electromagnetic properties of the system accurately; and it will be a generally suitable one.