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In re-entry, the drilling riser is set to connect with the seabed wellhead.Owing to the surface wave, the Mother Vessel (MV) and the drilling riser will oscillate in the ocean, which can cause the difficulty of the connection between the riser lower end and the seabed wellhead.To predict the riser response to the horizontal MV oscillation, the Flexible Segment Model (FSM) based dynamic calculation method is employed to make the numerical calculation.For the response of long drilling risers to the MV oscillation at different periods and amplitudes, there are 2 features easy to be neglected: the maximum response amplitude of riser lower ends and minimum oscillation location on the risers.Here, a 1500m drilling riser is taken as a typical long drilling riser.Its dynamic response to the horizontal MV oscillation is analyzed, the maximum response amplitude of riser lower end as the variation of MV oscillating period and amplitude is shown, and the Minimum Oscillation Point (MOP) on the riser is located.