Abstract: The marine unbonded flexible pipe is of better flexibility and adaptability with the special structure of relative independence and movability in each layer. It is the requisite pipeline for offshore development, especially for deepsea development, and it provides the important engineering application and strategic value for the development of deepsea resources. In this paper, the characteristics of marine unbonded flexible pipe were introduced and the main structure of typical pipe body were described. Then, the structural characteristics and functions of carcass layer, internal pressure sheath, pressure armor layer, anti-wear layer, tensile armor layer, intermediate layer and outer sheath were analyzed. Finally, the design features and working principles of three newly developed pipes were emphatically discussed, i.e., integrated production bundle, flexible pipe with anti hydrogen sulfide layer and flexible pipe with the carbon fiber tensile armor layer. It is shown that the marine unbonded flexible pipe is more suitable for deep-sea development than conventional steel pipes. The structure of marine unbounded flexible pipe at each layer can be designed according to the actual environmental requirements. These three newly developed flexible pipes become the research focus of marine unbonded flexible pipe owing to their satisfaction to the engineering demand of ultra-deepwater complex conditions.