Terahertz metamaterial biosensor is a label-free affinity sensor that enhances the strength of the local electromagnetic field. It is extremely sensitive to changes in the dielectric constant of the surrounding environment, thereby providing a new method of detecting micro or trace biological samples. In this work, a highly sensitive terahertz refractive index metamaterial absorber sensor for detecting the biological sample is proposed. The sensor consists of two concentric open metal rings and is a multimode resonator. With two independent adjustable operating bands in a frequency range of 0.7–2.5 THz, i.e. 1.079 THz and 2.271 THz, the sensor can observe different electromagnetic effects of the sample in the terahertz band. We evaluate the performance of terahertz sensors with indicators such as absorption characteristics and sensitivity. The sensor possesses the absorption higher than 99.9% in free space. In addition, the large Q value indicates that the sensor provides high frequency selectivity characteristics. Especially, the sensitivity of the sensor achieves 693.7 GHz/RIU, with a minimum refractive index change of 0.004 for the detection of biological samples, which provides good sensing performance. In the proposed sensor, a flexible material with low dielectric constant is used, which has the advantages of biocompatibility and portability and shows high stability at the 0°–60° oblique incidence angle and within 4% fabrication error. Moreover, the detection effectiveness of the sensor is verified by simulation experiments with ethanol-water mixtures. The sensor units designed in this paper have small interactions among them, work stably and are easily fabricated The sensor can significantly enhance the interaction between light and matter and has broad application prospects in terahertz high-sensitivity biosensing detection.