In a D-shaped twin-core fiber (DTCF), the central core is insensitive to the variation of the external environment, while the other core is highly sensitive. As an electro-optic polymer coated on a DTCF, the coupling between the two cores varies with voltages applied to the polymer. Based on this, a superior all-fiber modulator is proposed that bears little coupling loss, prohibits mode mismatch, and provides a more stable working circumstance. A half-wave driving voltage (Vπ) of 1.26 V is achieved. Moreover, a high modulation depth of 40 dB can be realized for a voltage of 2.7 V at a 1550 nm wavelength.