This paper presents a numerical analysis of lateral responses of a long slender marine structure under combined parametric and forcing excitations. In the development of the 3-D numerical program, a finite element method is implemented in the time domain using the Newmark constant acceleration method. Some example studies are performed for various water depths, environmental conditions and vessel motions. The relative amplitudes of combined excitations to a conventional forcing excitation are examined. The response amplitude of a combined excitation is much greater than that of a forcing excitation in the even number of instability regions of the Mathieu stability chart. The results demonstrate that a combined excitation needs to be considered for the accurate dynamic analysis of long slender marine structures subjected to a surface vessel motion.