Polymer nanocomposites (PNCs) have shown excellent mechanical properties owing to their combinations of nanoparticle (NP) and polymer properties, and they are now of significant interest in the fields of materials and soft matter science. Considering the difficulties in determining their multiscale dynamic characteristics, predicting the diffusion behaviors of polymers in polymer nanocomposites remains an open problem. In this study, the diffusion processes of polymers with different chain lengths in PNCs are numerically simulated via the dissipative particle dynamics (DPD) method, and the diffusion behaviors of polymer chains are described by a key dimensionless constraint factor χ, which is the ratio of the distance Lee from the first end of the polymer chain to the effective free diffusion length Lf. By analyzing the effects of the concentration of NPs, the lengths of polymer chains, and the interactions between polymer NPs on polymer diffusion, a scaling rate for the effective diffusivity of polymer chains is proposed with respect to the confinement factor χ, and it is confirmed that this scaling law can accurately predict the numerical results with a low confinement factor χ. This study provides a guide for the theoretical development and application of PNCs.