  We present an observation of zero to $\pi$ phase flips induced by Rabi
oscillation in an unbalanced Ramsey atom interferometer. The phase shift and
visibility are experimentally investigated by modulating either the
polarization or duration of Raman lasers, and they are well explained by a
theoretical model. In an atom interferometer, the $\pi$-phase flips are caused
not only by the sign of Rabi frequency but also by the Rabi oscillation.
Considering the $\pi$-phase flips, we propose the composite-light-pulse
sequences for realizing the large-momentum-transfer beam splitter and mirror,
which have the high immunity to the external phase noise in building the cold
atom interferometer.
