Changes in molecular weight and solubility of humic acid were investigated when hydrochloric acid, formic acid, acetic acid and propionic acid were spiked, by volumetric exclusion chromatography and TOC analysis. To verify the existence of charge-assisted hydrogen bonds (CAHB), formic acid was added into four kinds of model chemicals (i.e. ascorbic acid, benzoic acid, phenol, and catechol), and then excursion of their ultraviolet spectra were detected. Molecular weight of humic acid was reduced by the small molecular organic acids spiked. Additionally, solubility of humic acid significantly increased at pH approaching the p K_a of small molecular organic acids. All the findings demonstrate that CAHB possibly forms between small molecular organic acids and humic acid, which would destroy the supramolecules structure maintained by weak forces, thus leading to reduction of molecular weight and enhancement of solubility of humic acid. Moreover, the smaller the p K_a difference is, the stronger CAHB formed and the greater the molecule structure disturbance is. Small molecular organic acid could interact with some chemicals of humic acid bonded by CAHB when both have similar p K_a, and the energy for transition increases, which were indicated by UV spectrum and synchronous 2D correlation UV spectrum analysis.