Ultrafine noble metal nanoparticle (NP)-based catalysts exhibit excellent catalytic performance because of their highly dispersive nature and high utilization of metal active sites. Porous covalent organic polymers (COPs) which exhibit excellent stability and have abundant N atoms were considered as potential supporting materials for the immobilization of ultrafine metal NPs. Herein, highly stable azine-based COPs (ACOPs) were synthesized from 1,3,5-tris(p-formylphenyl)benzene and N2H4 center dot H2O under solvothermal reaction conditions. Highly dispersed ultrafine Pd NPs were successfully anchored on N-rich crystalline ACOPs by a simple reverse double-solvent strategy. The as-prepared Pd@ACOP catalyst exhibited excellent catalytic performance for the Suzuki-Miyaura cross-coupling reaction. This work offers a new method for preparing ultrafine metal NP-modified COP catalysts for sustainable catalytic applications.