A graphene-based metamaterial sensor working in the terahertz spectrum is proposed, simulated, and experimentally verified by measuring bovine serum albumin (BSA). Flexible, low-cost polyimide (PI) is used as the substrate, and aluminum with periodic square rings is chosen as the metal layer. Furthermore, the introduction of the graphene monolayer interacts with the molecules through π-π stacking, resulting in the highly sensitive detection of BSA by calculating the amplitude changes at the resonance frequency. The sensor, which is a biosensor platform that offers the advantages of a small size, high sensitivity, and easy fabrication, is a promising method for THz biological detection.