Heterogeneous Pd nanocatalysts are efficient catalysts for the Heck reaction but require multi-step, sophisticated procedures and harsh reaction conditions. In this work, a green and facile strategy has been developed to decorate Pd nanoparticles on polydopamine (PDA)-coated multi-walled carbon nanotubes (Pd/CNTs-PDA) via a one-pot method. The obtained nanoparticles were characterized by various techniques including transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, which proved that Pd NPs are well-dispersed on the PDA and between the surfaces of the PDA and CNTs. The resultant Pd/CNTs-PDA catalysts exhibit excellent catalytic reactivity toward the Heck reaction at low temperatures. Moreover, by DFT simulation, we found that during the PDA polymerization process, a large number of unsaturated [sbnd]N[dbnd] and C[dbnd]O species are more active than the groups on the PDA end product to anchor Pd NPs. The results provide evidence that the catalyst synthesized by the one-pot method exhibited good activity because sufficient active sites could be created to effectively promote Pd NPs dispersion during the dopamine polymerization process. Additionally, the Pd/CNTs-PDA catalyst was successfully employed in Heck cross-coupling reactions with various functionalized substrates. This method opens a window for the fabrication of high-performance nanocomposite catalysts under mild conditions using simple methods and has several potential applications.