To overcome the low temperature limitations during the overwintering period of bougainvillea in the Yangtze River Delta region and promote its application and popularization in this area, this study used bougainvillea ‘Rose Red’ as the experimental material. Treatments included a control group (CK, pure water), 0.39 mmol/L (low-concentration), and 0.78 mmol/L (high-concentration) hydrogen-rich water (HRW) irrigation. Physiological and biochemical indicators as well as morphological performance of leaves during the overwintering period were measured. The results showed that: (1) low-temperature stress significantly increased electrolyte leakage rate, malondialdehyde (MDA) content, and reactive oxygen species (ROS, including superoxide anion O₂^.- and H₂O₂) accumulation in bougainvillea leaves, with obvious leaf damage. (2) Low-concentration HRW treatment significantly increased chlorophyll content, maintained high photosynthetic capacity, promoted soluble sugar accumulation, reduced ROS generation and MDA accumulation, and decreased electrolyte leakage, thereby alleviating cold-induced damage. (3) High-concentration HRW treatment significantly aggravated the rise in the above injury-related indicators and intensified the damage caused by low-temperature stress. (4) Low-concentration HRW treatment enhanced cold resistance of bougainvillea by improving osmotic adjustment capacity and suppressing oxidative stress, while high-concentration HRW treatment produced the opposite effect. This study identified 0.39 mmol/L HRW as the appropriate concentration for improving the overwintering cold tolerance of bougainvillea, providing a new technical approach and theoretical basis for the large-scale application of bougainvillea in the Yangtze River Delta region.