A flexible metamaterial-based resonator was used to monitor the alcohol-water mixtures with different volume fractions using the terahertz time-domain spectroscopy (THz-TDS) in a transmission geometry. As the volume fraction of alcohol increases, the resonant frequencies of the biosensor blue-shift. The frequency shift of the second peak is higher than that of the first peak, the maximum shift value is 52 GHz, and the sensitivity is around 100 GHz/RIU. The developed resonator is flexible and has extremely low solution consumption (≈20 μL), indicating the proposed method shows possibility for the applicability of THz-TDS and the industrial application of THz resonators in the field of biology.