Chilling damage in Heilongjiang Province is an important agricultural meteorological disaster that leads to unstable maize yield and low quality in this region. However, there are few studies on the ability and degree of antioxidant stress in maize under low-temperature stress. Mass concentration of humic acid were 0.2%, 0.4%, 0.8%, 1.2%, 1.6% and 2.0%, and water as the control, the changes of activities of representative antioxidant enzymes, ascorbic acid-glutathione (AsA-GSH) cycle, systemic resistance and carbon-nitrogen metabolism of maize seedlings under low-temperature stress of day and night for 48 h at 15℃/8℃ were studied to illustrate the tolerance mechanism of humic acid for low-temperature stress and the appropriate concentration of soaking seed. The results showed that soaking seed with humic acid was mainly induced by H₂O₂ accumulation under low temperature stress. With the increase of seed soaking concentration, most of the activity of antioxidant enzymes and system resistance enzymes, the AsA-GSH cycle and main substance content of carbon and nitrogen metabolism, root activity and fresh weight showed unimodal curve distribution. The soluble protein content of leaves and roots increased by 23.3%-34.9% and 31.9%-89.6% at the concentration of 0.8%-2.0%, respectively. The activity of phenylalanine ammonia-lyase increased by 16.8% and 38.8%, the activity of polyphenol oxidase increased by 27.7% and 13.4%, the content of ascorbic acid increased by 62.6% and 55.3%, and the content of dehydroascorbic acid increased by 31.9% and 66.4% in leaves and roots at the concentration of 1.6%, respectively. The activity of hydrogen peroxide in leaves and roots increased by 66.4% and 108.1% at the concentration of 0.8%. Therefore, H₂O₂ content in leaves and roots after soaking seed significantly reduced by 18.7%-37.6% and 27.5%-49.7% compared with the CK, respectively. The fresh root weight increased significantly by 85.2% and 105.3%, and the fresh weight of single plant increased significantly by 31.9% and 40.8% at the concentration of 0.4% and 0.8% compared with the control, respectively. In this study, the treatment at the concentration of 0.4%-0.8% enhanced the operation activity and systemic resistance of the AsA-GSH circle system with AsA as the core, improved the antioxidant stress ability of maize seedlings, and obtained the maximum root activity and fresh weight of plant, which were a relatively appropriate seed soaking concentration under hydroponic condition.