【Objective】Rice-upland crop rotation is one of the main cropping systems in the middle and lower reaches of the Yangtze River in China. During the rotation process, acidic paddy soils generally show a rise in pH during flooding for rice cultivation and a decrease after drainage. This phenomenon is mainly driven by redox reactions involving inorganic compounds such as iron/manganese oxides and sulfates. Organic reductants, however, also contribute significantly to pH dynamics during the flooding-draining cycle through their redox interactions with oxygen. Thus, this study aimed to investigate whether incorporating green manure during flooding to increase organic reductant production could suppress the pH decline upon draining and reduce the risk of acidity stress on upland crops. 【Method】Three acidic paddy soils, collected from Jinxian, Jiangxi; Binyang, Guangxi; and Ji''an, Jiangxi, were selected as test soils, and Chinese milk vetch (Astragalus sinicus L.) was used as the amendment material. Four treatments with mass ratios of 0, 0.5%, 1%, and 2% of the Chinese milk vetch were established in a short-term flooding-draining incubation experiment. The first two soils from Jinxian and Binyang were used to evaluate the ameliorative effect of Chinese milk vetch addition on soil acidity after the draining phase, while the soil from Ji''an was employed to further investigate the improvement in soil pH and the underlying mechanisms. In addition, an oxidation experiment using anaerobic incubation solutions of Chinese milk vetch with four acidity gradients was conducted to further elucidate the mechanisms involved. 【Result】The results showed that after the flooding-draining treatment, pH values of the two soils in the treatments with Chinese milk vetch addition were significantly higher than those of the control and increased with the increasing amount of Chinese milk vetch applied. Compared with the control, the pH values of the paddy soils from Jinxian and Binyang under the 2% Chinese milk vetch addition were elevated by 1.66 and 1.14 units, respectively. Meanwhile, the treatment also decreased the contents of soil exchangeable acidity and exchangeable Al3+ after the drainage of the soils, with reductions of more than 23% for each. Further investigation on the flooding-draining process with the soil from Ji''an indicates that the redox reactions of inorganic reducing substances were essentially reversible; meanwhile, the addition of Chinese milk vetch significantly increased the content of organic reducing substances in the soil during the flooding period and promoted their oxidative consumption during the subsequent draining process. Compared with the control, the amounts of total organic reducing substances consumed in the treatments increased by 0.84, 2.91, and 12.50 cmol·kg-1, respectively. A separate incubation experiment with Chinese milk vetch alone further confirmed that the anaerobic incubation solution of the Chinese milk vetch, which is rich in organic reducing substances, exhibited a significant increase in pH upon aeration, and that external acid addition further enhanced the H+ consumption during the oxidation process.【Conclusion】The above results indicate that the oxidation of organic reducing substances is an important mechanism for inhibiting the decline in soil pH during the draining period, and that the reduction process of dissolved oxygen in the soil solution also contributes to proton consumption. Based on this finding, the soil acidity in the dry season under a rice-wheat (canola) rotation system can be effectively regulated, mitigating the harm of soil acidity and providing a new perspective for the amelioration of acidic soils.