Abstract: For the purpose of shaping the low frequency terahertz Gaussian beam into the large-size flat-topped beam, a method of designing a large-size flat-topped beam shaping system using dual-phase plate structure was proposed. Based on the Input-Output algorithm, the system parameters were calculated using angular spectrum theory, and the designed system was numerically simulated and the corresponding experimental system was built to verify whether the lateral intensity distribution of the emitted light field met expectations. The results show that the designed dual-phase plate shaping system can successfully convert the 0.1 THz Gaussian beam into a 100 mm-size flat-topped beam. The simulation results are close to the theory, which proves the rationality of the designed system. In the experiment, the beam diameter reaches 228 mm. Although there is some energy leakage, the flatness is very good, and the relative root mean square is even better than the theoretical and simulation results, reaching more than 89%, which fully demonstrates the actual effectiveness of this design method.