Significant research interest has been directed toward terahertz (THz) metamaterials, motivated by their prospective applications in the domains of biosensing and environmental detection. Among their most beneficial properties are the capabilities for speedy and non-destructive analysis. This study demonstrates a terahertz plasmonic sensor for monitoring environmental refractive index. The structure is designed with three key layers: a top gold film etched with two elliptical cross-shaped resonators, a middle silica insulator, and a continuous gold base layer. The novelty of this architecture lies in the specific symmetry of the crossed elliptical resonators, which suppresses radiative losses to achieve an exceptionally high-Q resonance—approximately eight times superior to traditional THz metamaterial sensors. We employed a 3D finite element model in COMSOL Multiphysics^® to simulate the absorption spectra and analyze the field distribution. Notably, the model demonstrates high-performance resonance, with a peak absorption of 88\% occurring at 3.684 THz. The resonance condition, which is critical for sensing, is facilitated by the coexistence of electric and magnetic dipole responses, and the performance is subsequently determined by the associated localized field distribution. To verify the underlying physical mechanism, the geometrical parameters of the sensor were systematically varied, and the corresponding absorption performance was analyzed. The design also achieves high-performance refractive index sensing with a sensitivity of 2.02\text{T}\text{H}\text{z}/\text{R}\text{I}\text{U}, a \text{Q}-\text{f}\text{a}\text{c}\text{t}\text{o}\text{r} of 584.01, and a \text{F}\text{O}\text{M} of 321.14. These findings indicate that the presented sensor holds considerable promise for future deployment in biomedical detection and the monitoring of essential metrics within environmental applications.