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<p>Humic-like substances (HULIS) are complex macromolecules in water-soluble organic compounds (WSOCs) containing multiple functional groups, and transition metal ions (TMs) are ubiquitous in atmospheric particles. In this study, potential physical and chemical interactions between HULIS and four TM species, including Cu<span class="inline-formula">²⁺</span>, Mn<span class="inline-formula">²⁺</span>, Ni<span class="inline-formula">²⁺</span>, and Zn<span class="inline-formula">²⁺</span>, were analyzed by optical method under acidic, weakly acidic, and neutral conditions. The results showed that Cu<span class="inline-formula">²⁺</span>, Mn<span class="inline-formula">²⁺</span>, and Zn<span class="inline-formula">²⁺</span> only slightly enhanced mass absorption efficiency (MAE<span class="inline-formula">₃₆₅</span>) of HULIS in winter and had indiscernible effects on the absorption Ångström exponent (AAE) of HULIS in both seasons under all acidity conditions. All four TMs had fluorescence quenching effects on winter HULIS, and only Cu<span class="inline-formula">²⁺</span> had similar effects on summer HULIS, with the highest quenching coefficients found under weakly acidic conditions in both seasons. The <span class="inline-formula">¹</span>H-nuclear magnetic resonance (<span class="inline-formula">¹</span>H-NMR) and Fourier-transform infrared (FTIR) spectra revealed that Cu<span class="inline-formula">²⁺</span> mainly bound with aromatic species and tightened the molecule structures of HULIS. The parallel factor analysis (PARAFAC) results extracted four components of HULIS, including low-oxidized humic-like substances (C1), N-containing compounds (C2), highly oxidized humic-like substances (C3), and the mixing residuals (C4), from the fluorescence spectra in both winter and summer. The spectral characteristic of HULIS with Cu<span class="inline-formula">²⁺</span> additions under three acidity conditions indicated that electron-donating groups of HULIS mainly corresponded to C1 and C3, with Cu<span class="inline-formula">²⁺</span> binding with HULIS by replacing protons, while electron-withdrawing groups of HULIS could correspond to C2, with its connection with Cu<span class="inline-formula">²⁺</span> through electrostatic adsorption or colliding-induced energy transfer.</p>