The Three Gorges spillway dam section with a maximum dam height of  181.0 m , is one of the important constructions in the Three Gorges Project. The previous seismic investigations on the spillway dam in the past years did not take into account the longitudinal construction joints of the dam body. On the basis of the actual dam with longitudinal joints, the contrast state of the joints in the seismic response of the dam under earthquake has been carried out.The investigation included three different dam structure models: the dam without longitudinal joints, the dam with joints but without joint gap, and the dam with joints and with joint gap. The contact property of the gap was assumed to be nonlinear and a time step-by-step integral method was adopted.The results given in this paper indicates that the seismic responses of the spillway dam with or without joints are different. The dam with joints in performance seems like a three overlapping beam,in which the stress is discontinuous at two ends of the joint. However, the dam model with longitudinal joints still satisfies the seismic design requirements as the dam in its entirety.