We propose and analyze theoretically an asymmetric Ge/SiGe coupled quantum well (CQW) for silicon based optical phase modulator. An 8-band k⋅p model is used to calculate the eigenstates and absorption spectra of the CQWs. The simulation results exhibit unique physical characteristics owing to the coupling between the two wave functions through the thin barriers. We can achieve an electro-refractive index variation as high as 9 × 10-3 at the wavelength of about 1461 nm under the electric field of 30 kV/cm. The product VπLπ of half-wave voltage and length of phase shift region is estimated to be 0.01 V cm. The proposed asymmetric Ge/SiGe CQW scheme provides a promising candidate for high speed, low voltage, low power consumption and compact optical phase modulators in silicon-based integrated optoelectronic devices.