Radioactive iodide is a major hazardous volatile species released during nuclear accidents, posing significant challenges for capture under water vapor and dynamic conditions. In this study, Ag-modified zeolite (Ag/NaY) was synthesized by an ion-exchange method, and its iodine capture performance was systematically investigated in both powder form and as a functional washcoat for microchannel reactors. Ag/NaY were evaluated for the capture of iodine species from aqueous iodine solution, iodine vapor, and methyl iodide (CH₃I) vapor. The results demonstrated that Ag/NaY exhibited excellent iodine adsorption capacities, which was attributed to the strong chemical affinity between Ag active sites and iodine species. Ag/NaY was deposited onto cordierite honeycomb substrates through an impregnation process to construct Ag/NaY functional washcoat for microchannel reactors. Dynamic adsorption experiments revealed that the coated microchannel reactor maintained a CH₃I removal efficiency above 95% over 32 h under high humidity conditions. SEM, XRD, and XPS analyses confirmed the uniform dispersion of Ag species and the formation of stable AgI through chemical fixation of iodine. Furthermore, simulation results based on Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) calculations elucidated the preferential adsorption sites of CH₃I and H₂O molecules within zeolite supercages, revealing the competitive adsorption mechanism under humid environments. This work establishes a multiscale strategy from Ag-modified zeolite design to functional washcoat construction and engineering application, providing an efficient and practical approach for radioactive iodine capture in nuclear emergency environments.