According as three motion modes of motion process of the seated large diameter cylinder breakwater(LDCB) under random waves , the dynamic model system composed of spring-damper-antislider to control the lateral sliding is introduced, and corresponding dynamic analysis space model is established. The space model is realized by Parameter Design Language APDL of the large finite element software ANSYS. Calculated results by the space model comparing with those by the plane model respectively , some valuable conclusions are achieved. The validity of the model, which can be used in dynamic response analysis of the seated LDCB under random waves, is confirmed by the given example.