Dielectric characterization of materials is crucial for accurately assessing their terahertz electromagnetic properties and ensuring their effectiveness in relevant applications. A method is proposed for dielectric property testing of Materials Under Test(MUT) based on their self-resonant characteristics, combining electronic terahertz technology with waveguide aperture reflection. The electromagnetic process and influencing factors of the self-resonance of the MUT at the rectangular waveguide aperture are clarified using the Finite-Difference Time-Domain(FDTD) method for electromagnetic field numerical analysis. Subsequently, the relationship between the waveguide reflection coefficient and the dielectric properties of the MUT is obtained. Based on the frequency shift of the material's self-resonance, grooves and etched channels are created on a single-crystal silicon wafer which is then bonded with polydimethylsiloxane(PDMS) to measure the dielectric properties of the liquid within the channels. Finally, a testing system with a single-crystal silicon wafer as the MUT is constructed in the laboratory according to the simulation rules, verifying the reliability of the simulation results and the feasibility of the proposed dielectric testing method.