The measurement of radar cross section (RCS) of scale models at terahertz can decrease the cost and time of the experiments, so it attracts wide attentions. In real measurement, generally a Gaussian beam or a similar beam source is adopted，while in the RCS estimation, usually a plane wave is assumed as the incident beam. In order to study the error caused by the assumption, the RCS of a perfect conducting flat plate is estimated in two dimension when the incident beam is a Gaussian beam. In the estimation, the influence of 2.52 THz collimated laser on RCS is studied. The estimation results show that choosing beam width of 40 mm can help to keep the amplitude of the error around 0.3 dB when the width of the flat plate is 20 mm.