The Radar Cross Section(RCS) is an important physical quantity for measuring the target's ability to scatter radar waves. It plays an important role in target recognition and imaging. A new calibration method is proposed and software distance gate technology is utilized to improve the measurement accuracy of the target RCS based on the 440 GHz terahertz target RCS measurement system. The cylinders with different roughness are measured to obtain the RCS measurement results. Compared with the theoretical values, it is found that using the new processing technology can achieve a high level of accuracy, which can be applied to measure RCS of complex targets and study the law of scaling.