Heteroatom-doped carbon materials have attracted immense attention for heterogeneous catalysis applications because of their excellent physicochemical properties. In this work, we prepared a cost-effective triazinyl-containing two-dimension COFs (2D-COFs), and then the 2D-COFs was self-templated carbonized to fabricate 2D nitrogen-doped carbon nanosheets (NCNs). NCNs were modified with highly dispersed small palladium nano particles (Pd NPs), affording the Pd/NCNs catalyst. The as-prepared NCNs and Pd/NCNs exhibited a smoke-like morphology, a large specific surface area, and mesoporous and macroporous structures, which can enhance reactants diffusion and mass transfer. The obtained Pd/NCNs catalyst exhibited excellent activity, stability, and recyclability in the aqueous-phase hydrodechlorination (HDC) of organic hazardous substances chlorophenols (CPs). Meanwhile, outstanding activity and high selectivity were observed for the subsequent hydrogenation of the HDC product phenol to cyclohexanone. The excellent activity of the Pd/NCNs catalyst was related to the synergistic effect of Pd NPs and the nitrogen atoms doped in the NCNs. Thus, this study offers a novel, facile route for fabrication heteroatom-doped carbon materials derived from cost-effective COFs materials to prepare highly dispersed small noble-metal-NP-modified catalysts for various catalytic applications.