Abstract: The near-field scattering characteristics of dielectric targets are studied in the terahertz band based on the generalized Kirchhoff impedance boundary conditions and the physical optics method. The formula of the near field scattering for dielectric targets is deduced. In the light of the increase in calculation amount caused by the shorter wavelength, a fast computational method using surface element as the calculating unit and pixel as occlusion judgment unit is proposed for the near field scattering computation in terahertz band. The method ensures the calculation accuracy and reduces greatly the computational complexity and the time consumption of occlusion judging. The calculation of the near-field Radar Cross Section (RCS) produced by a dielectric cylinder and a duck mouth scatterer is performed in terahertz band. Meanwhile, the effect of the phase on the near field RCS in different distances and frequency is analyzed.