Terahertz technology is widely used in the radar cross section (RCS), because the wavelength of terahertz can help downsize the model to a proper range and the measurement of scale model can help decrease the cost of time and the expenditure of the experiment. In the calculation of the RCS of a rough surface, sometimes, the question is deduced into the calculation of the boss on a flat plate and the monostatic radar is the one that mass of radar system adopted. However, a plane wave is usually assumed in most of the RCS estimation, while in real measurements, the energy of the incident beam obeys the Gaussian distribution. In the estimation, the influence of 2.52 THz collimated Gaussian beam on monostatic RCS is studied in details and the relatively error is analyzed.