Abstract: Long flexible cables is one of important parts of complex systems used in explorations and exploitations of ocean resources, particularly in deep or even ultra-deep water. These flexible cables, with large aspect ratio usually at level of 10³, need to be installed with distributed buoyancy modules along its body length. In that case, these distributed buoyancy modules make deep-sea cable configuration more complex and, moreover, their structural properties, such as structural tension and mass, are axially-changing. Thus structural motion response and its spatial-temporal evolutions become more complicated, which brings serious challenges to structural safety. In this study, a novel structural configuration, i.e. the double-stepped cable, is considered, and the dynamic governing equations of deep-water cable with distributed buoyancy modules are developed, principally based on the particular fluid-solid interaction characteristics and its coupling representation of the loadings, along with the experimental observations and verifications using our experimental water tank. The numerical simulations of the double-stepped cable dynamic response are carried out using the modified finite element approach. The responses and its corresponding propagations of the double-stepped cables, in terms of structural displacement and tensions along cable length, under environmental loadings and top-end excitations are comprehensively examined. In addition, the evolutions of displacement amplitudes and wavelengths of this kind of structure with axially-varying tension are explained based on the WKB theory. Our results show that the response does not change monotonously as it propagates along the cable length, and a local peak value may appear in the region with lower tension. Owing to the distributed buoyancy modules, along with axially-varying and discontinuous structural properties, the response spatial-temporal evolutions becomes more variant. There are mixed effects coming from both standing wave and traveling wave. It is also found that structural tensions not only affect the response amplitude significantly, also cause changes of wavelength during the process of response propagation.