To fulfill the requirements of the dielectric property measurement in the terahertz band, herein, a broadband quasi-optical system was designed and verified utilizing a planar scanning system. Additionally, the method of retrieving the dielectric parameters was discussed. Our experimental findings indicated that the measurement results were in good agreement with the theoretical results. Boron silicon, and deionized water were used for verifying the measurement, and the permittivity was obtained using a numerical method. We found that the dielectric properties were in good agreement with the typical values. This indicated that the proposed quasi-optical method effectively characterized the permittivity.