Resin matrix composites are widely utilized in a variety of typical load-bearing and non-load-bearing constructions in fields such as aeronautics and aerospace. With structural and functional integration, traditional off-line monitoring approaches can no longer match composites’ sophisticated expectations. With the ongoing development of structural health monitoring (SHM) of composite materials, flexible sensors primarily composed of conductive particles such as carbon nanotubes, graphene, and MXene have opened up new possibilities for early damage detection, early warning of service status, and intelligent diagnosis of composite materials. This paper primarily undertakes a systematic evaluation of the structural health monitoring technology of flexible sensors, focusing on introducing the design ideas, essential technologies, mechanism mechanisms, and application advancements of flexible sensors, and looking ahead to their future applications.