The heave of the floating platform induces a fluctuation in time of the axial tension of the riser. A possible and undesirable phenomenon is the excitation of a transverse riser vibration caused by this fluctuation. Parametric vibration might destabilize the straight equilibrium of the riser and cause it to vibrate at a dangerously high level. Tension in the riser is typically assumed to be constant over the length, but tension actually varies linearly from the top to the bottom due to the effect of gravity which leads to coupling between the modes. Firstly, the governing motion equation of deep-water risers under harmonic parametric excitations was formulated. Then, considering the modes coupling, stability behavior of the riser with and without damping was investigated by applying the Floquet theory, and dynamic response under critical parameters was obtained. The results showed that instability zone for coupled system was significantly increased, and a small excitation could produce large responses, especially the bending stress would increase significantly.