Abstract: The annulus state of flexible oil and gas marine risers is an essential factor that afects their integrity management. Herein, the domestic and international research progress on the annular state of flexible risers was summarized based on the flexible riser with multilayer composite structure. Meanwhile, the subsequent key research directions were proposed from the following three aspects against the deficiencies of the existing research results, i.e. gas permeation behavior of lining material, theoretical prediction model of annular state, monitoring and detection of annular state. (1) The permeation behavior could be analyzed by preparing specimens with the raw material of flexible riser by the same processing technology, and the lining material of scrapped flexible risers could be studied to master the permeation behavior and law of materials throughout the whole life cycle. (2) The permeability coefficient of similar materials could be adopted in the annular state prediction model, but the dynamic impact of material aging on permeability behavior should be considered and compared with the prototype experiment of flexible riser. (3) The design of the joint at the bottom of riser should be improved to increase the remote and local pressure monitoring structures at the bottom of annulus and set the operation interfaces of remotely-operated vehicles, so as to assure the prediction accuracy and maintenance efectiveness of annular state.