A theoretical investigation and simulation of a terahertz metasurface refractive index sensor composed of graphene-metal is presented in this article. The analysis and evaluation of the reflector's performance is conducted through the implementation of the frequency-domain finite-difference method. The transmission and reflection spectra are analyzed in order to identify the characteristic dimensions that can enhance both sensitivity and reflectivity by optimizing the material's thickness, cycle duration, and width of the etching structure. The findings indicate that the sensor exhibits a quality factor of 8.4 and a maximal refractive index sensitivity of 1.48 THz/RIU within the operating frequency range of 0.1-2.0 THz. These values are three times greater than those of the conventional sensor. The design of this refractive index sensor has the potential to revolutionize bioassays pertaining to the quantification of proteins, viruses, cancer cells, and their markers.