Un-bonded flexible pipes are a key driver for oil and gas production and delivery in ultra-deepwater. They are used to transport produced fluids between floating production systems and submarine wells. With oil and gas production in ocean depths of more than 3,000 meters, failures have become a major challenge for flexible pipes. There are many failure modes of un-bonded flexible pipes, and domestic and foreign experts and scholars have done a lot of research on the failure of corresponding functional layers, but the failure modes of the metal layers about flexible pipes is lack of a relatively comprehensive review. Based on the integration of nine common failure modes of un-bonded flexible pipes proposed by the specification API RP 17B, the research progress of six common failure modes of flexible pipes metal layers at home and abroad is reviewed in detail. In addition, the existing problems and shortcomings of metal layers in the current research on the failure mechanisms of flexible pipes are discussed. In order to lay a foundation for structural innovation and integrity management of flexible pipes, the next step of failure modes research on flexible pipes is proposed.