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The combustion of n-heptane/methane mixtures in dual-fuel systems presents a promising strategy for reducing soot emissions, yet the underlying suppression mechanisms require further quantification.This study employs numerical simulation to investigate the structural and effects of methane addition on a laminar n-heptane diffusion flame.A validated coupled model, incorporating detailed chemical kinetics mechanism and a discrete sectional soot model, is applied.Simulations results reveal that methane blending significantly suppresses soot volume fraction, with the normalized peak value decreasing monotonically with methane addition.While the peak flame temperature remains stable, the entire thermal structure shifts toward the fuel nozzle due to altered mixture reactivity.Analysis of key intermediates shows that methane addition reduces peak concentrations of aromatic precursors, with a progressively stronger inhibition effect observed for larger polycyclic aromatic hydrocarbons.This size-dependent suppression indicates that methane chemically interferes with molecular growth processes beyond the first aromatic ring, likely through perturbation of the H-abstraction-CH-addition (HACA) mechanism and radical pool modification.The results elucidate the combined dilution and chemical effects governing soot reduction, providing critical insights for optimizing dual-fuel combustion towards minimal particulate emissions.