Abstract: To elucidate the underlying mechanism behind the anomalously high proportion of NO₂ in NO_x in methanol/diesel dual-fuel engines and improve the accuracy of NO_x emission predictions， a multi-component diesel representation strategy coupled with targeted optimization of key reaction pathways was proposed. A compact chemical kinetic mechanism that integrates methanol low-temperature oxidation， combustion of three diesel surrogates （n-hexadecane， iso-hexadecane， and 1-methylnaphthalene） and the full NO_x formation and conversion chemistry was developed. Validated against multi-source experimental data from shock tubes and rapid compression machines， the mechanism demonstrates excellent predictive fidelity across a wide range of temperatures， pressures， and equivalence ratios for ignition delay time， laminar flame speeds， and concentrations of key intermediate species. A three-dimensional computational fluid dynamics（CFD） simulation model was established based on CONVERGE， incorporating the newly developed coupled methanol-diesel kinetic mechanism. Analysis of multi-parameter spatiotemporal evolution maps during the core combustion period reveals that， under a 20% methanol substitution rate， the significant rise in NO₂ fraction is primarily driven by HO₂ radicals continuously generated via methanol low-temperature oxidation. These HO₂ radicals promote the conversion of NO to NO₂ through the dominant reaction NO+HO₂NO₂+OH. Concurrently， the evaporative cooling effect of methanol establishes a low-temperature premixed zone， which combined with the distinct roles of the three diesel components， and then creates a synergistic dual-effect mechanism. The n-alkanes supply precursors for thermal NO formation， iso-alkanes delay H₂O₂ decomposition， sustaining HO₂ availability， and aromatics consume OH radicals， thereby suppressing the reduction of NO₂ back to NO. This synergy simultaneously enhances NO₂ production and inhibits its reduction， leading to a markedly elevated NO₂/NO_x ratio in the dual-fuel mode.