Objective Acid precipitation, a common method in plant protein processing, can efficiently achieve protein enrichment and removal of non-protein components (including off-flavor substances) on a large scale. This study aims to explore the causes for the abnormal increases in aldehydes of soymilk after acid precipitation.Method Changes in volatile compounds of soymilk before and after acid precipitation were analyzed by gas chromatography-mass spectrometry (GS-MS). Soymilk components were fractionated by centrifugation, and the contributions of different fractions to the increases in aldehydes were investigated. The changes in the molar concentrations of lipid hydroperoxides before and after acid precipitation were determined, and a molar ratio between these changes and the alterations in aldehyde content was established. Subsequently, the effects of critical factors (acids, acid precipitation temperature, and Fe²⁺ concentration) influencing lipid hydroperoxide decomposition on the mass fraction of hexanal in the product after acid precipitation were systematically examined.Result The changes in the mass fractions of alcohols, acids, ketones, and furans in soymilk after acid precipitation were as anticipated. The mass fractions of aldehydes (including hexanal, nonanal, and trans-2-heptenal) increased, while the mass molar concentration of lipid hydroperoxides demonstrated a remarkable decrease. The centrifugation results showed that the oil phase in the upper layer was the main source for the increases in the mass fractions of aldehydes, with the largest contribution. The subsequent analysis revealed that the ratio of lipid hydroperoxide molar concentration decrease to aldehyde molar concentration increase was 5∶1 following acid treatment, indicating partial decomposition of the former into aldehydes, which is consistent with previous findings. The analysis of influencing factors showed that compared with hydrochloric acid, citric acid or malic acid for acid precipitation reduced the decomposition of lipid hydroperoxides and increased the formation of hexanal. However, higher acid precipitation temperature and higher Fe²⁺ concentration promoted the decomposition of lipid hydroperoxides and the increase of hexanal accumulation.Conclusion During acid precipitation, the abnormal increases in aldehydes exhibited a distinct distribution pattern compared with other off-flavor substances. Replacing hydrochloric acid with citric acid or malic acid, lowering the precipitation temperature, and controlling the Fe²⁺ concentration within the system can effectively inhibit the decomposition of lipid hydroperoxides, thereby reducing the formation of aldehydes.