Graphene-mediated surface-enhanced Raman scattering (SERS) substrates are currently being investigated for their potential in ultrasensitive detection. Graphene (G) combined with plasmonic metal nanostructures is expected to provide an exceptional SERS response. Here, we prepared a flexible multilayer composite structure by directly decorating silver nanoparticles (Ag NPs) on CVD graphene/copper foil under ultrasonic conditions. Due to the multi-dimensional plasmonic coupling of precious metals, extra chemical enhancement and exceptional molecule harvesting capability of G interlayer, the as-prepared Ag/G/Cu multilayer composite structure manifested ultrahigh sensitivity with the detection limit of Rhodamine 6G (R6G) as low as 1.0 × 10-14 M and a high enhancement factor (EF) of 8.86 × 108. The high performance flexible SERS substrate, with its ingenious process and simple structure, demonstrates excellent potential for applications in environmental protection.