A method and apparatus for adjusting a modulation index of an analog optical signal using an interference phenomenon of a coherent light, the method including dividing an analog optical signal for a first path and a second path, extracting an optical carrier from an analog optical signal divided for the first path, controlling an intensity and a phase of the optical carrier, and controlling a modulation index of the analog optical signal by combining an analog optical signal divided for the second path and the optical carrier of which the intensity and the phase are controlled.