In this study, we propose and design the structure of a graphene-based D-shape twin-core fiber (DTCF) modulator by utilizing the unique absorption characteristics of graphene and the coupling between two cores, which integrates the advantages of absorption modulation and coupling modulation. Further, the modulation feasibility of this structure can be verified using numerical simulation. Subsequently, the effects of the transmission mode, two core radius, twin-core distance, number of graphene layers, and transition layer materials of the modulator on the performances are analyzed based on the finite element method. Finally, a high-performance modulator with a modulation bandwidth of 8.557 GHz, an extinction ratio of 67.64 dB, a half-wave voltage of 1.763 V, a modulation depth of 98.75%, an insertion loss of 1.4 dB, and a modulator length of 3.07 mm can be achieved through optimization.