This study investigated a polyethylene terephthalate (PET) substrate and the effect of indium tin oxide (ITO) thin-film interference on the electromagnetic resonance of distorted metamaterials. The photoresist was developed on a PET substrate and swollen using isopropyl alcohol. The SRRs had various total lengths, gaps, and line widths. In addition, each of these three dimensions varied greatly and thus the distorted SRRs exhibited a broadband resonance spectrum. An ITO thin film was coated on the back of the PET substrate with a distorted metamaterials sample, and the terahertz spectrum was measured. The experimental results revealed that the ITO thin film can flatten the spectrum of the SRR sample. To determine the underlying reason, we varied the sheet resistance of the ITO film and observed the differences among the corresponding spectra. The flattened spectrum of the ITO films enhanced the thin-film interference effect of the PET substrate; consequently, the distorted metamaterials exhibited a flattened spectrum. These distorted metamaterials can be applied in terahertz imaging, terahertz communication systems, and optoelectronic integrated circuits.