The free-space method is a analysis method applicable for measuring electromagnetic parameters of materials, owing to its non-contact and non-destructibility. Base on the resonance property of reflectivity under sweep frequency mode, a novel free-space method is presented. For the study of constructive interference or destructive interference, the method is used to measure the real component of complex dielectric constant through the thickness of material and the interference frequency. The method can also measure the imaginary component of complex dielectric content by the positions of peaks and valleys of waveform or/and their correlation. The study also demonstrates the higher precision obtained by using the valley-to-valley ratio than by using the standing waveform ratio (peak-to-valley ratio).