Surface-enhanced Raman scattering (SERS) sensors based on gold/silver nanoparticles (Au/Ag NPs) play a pivotal role in biomedical diagnostics, environmental monitoring, and public security due to their ultrahigh sensitivity, broad dynamic range, mature surface modification techniques, and tunable plasmonic hotspot distribution. Compared to traditional rigid substrates, flexible SERS substrates offer broader applicability owing to their conformability to complex surface morphologies. However, achieving strong bonding force between Au/Ag NPs and flexible materials and controllably constructing plasmonic hotspots on flexible materials still face challenges—which affect the structural stability and detection sensitivity of sensors. This review first summarizes two typical SERS enhancement mechanisms: electromagnetic enhancement and chemical enhancement mediated by Au/Ag NPs. Subsequently, the physical and chemical integration strategies for Au/Ag NPs with flexible materials are systematically summarized, including dip-coating and impregnation, physical vapor deposition (PVD), interface self-assembly, in-situ growth, and electrochemical deposition. The advanced strategies developed based on these methods for achieving strong interfacial bonding and constructing high-density plasmonic hotspots are critically analyzed. In addition, a comparison of key performance metrics such as sensitivity, stability, cost, and scalability is conducted. Following this, to address the challenges encountered in practical applications, this review elaborates on the development of Au/Ag NP-based flexible SERS sensors in typical applications such as food safety, health monitoring, public safety, and environmental pollutant tracking. Finally, we outline challenges and future directions in developing high-performance Au/Ag NP based flexible SERS sensors, providing a valuable reference for advancing this field.