In order to improve the performance of modern rockets, large area-ratio nozzle s are adopted. Over-expansion occurs in large area-ratio nozzles due to high back pressure at low altitudes, leading to unsteady and asymmetrical flow separation inside the nozzle, as well as severe side-loads phenomena. As a result, the large side-loads may lead to remarkable problem of fluid/structure interaction. In this paper, the situation and progress on nozzle flow separation is reviewed. Research object and product of different period were described. Several factors that induce side loads are enumerated, including transition of shock pattern, declining separation line, pressure pulsation, buffeting, fluid/structure coupling, etc. On this basis, progress and research methods on fluid/structure interaction are further analyzed, including experimental and numerical methods. Besides, new research objects that are probably related to side loads are pointed out. Finally,several corresponding critical mechanical problems which need to be considered and settled during large area-ratio nozzle design are discussed, and the suggestion about critical aspects of side-loads needing to be further studied is put forward.