A new type of core-shell graphene@poly(dopamine)-Ag nanoplatelets was developed to improve the dielectric properties of thermoplastic polyurethane (TPU) nanocomposites. Dopamine was oxidatively polymerized to form a polydopamine (PDA) layer on the surface of graphene nanosheets (Gns), and then Ag nanoparticles were chemically reduced by silver nitrate and uniformly embedded on the surface of PDA. This obtained Gns@PDA-Ag core-shell nanoplatelets were filled into TPU to prepare a flexible dielectric material. The microstructure and dielectric performances were tested. Results indicated that the Gns@PDA-Ag core-shell nanoplatelets were evenly dispersed in TPU matrix. Moreover, the addition of Gns@PDA-Ag core-shell nanoplatelets could effectively improve the dielectric constant of the composite. Under low-frequency test conditions, the dielectric constant of the TPU/Gns@PDA-Ag (4 wt%) nanocomposite was 117.81, which was about 14 times higher than that of pure TPU. Theoretical analysis shows that the PDA shell not only promoted the uniform dispersion of Gns in TPU matrix, but also enhanced the interface bonding between the nanoplatelets and the TPU matrix. Moreover, introducing ultra-small Ag nanoparticles had effectively decreased the dielectric loss and conductivity of TPU/Gns@PDA-Ag composites due to its coulomb blockade and quantum trapped carrier structure effects, which resulted in the improvement of energy storage characteristics of composite.