Oat cultivar ‘Yanke 2’ was used as experimental material. Four treatments, namely normal water supply (CK), spraying humic acid soluble fertilizer (CKH) under normal water supply, severe drought stress (SS) and spraying humic acid soluble fertilizer (SSH) under severe drought stress, were set in pot. We determined reactive oxygen species (ROS) level, antioxidant enzyme activity, total antioxidant content and yield in oat leaves to clarify the regulation effect of humic acid watersoluble fertilizer (HA) on ROS scavenging system in oat leaves under severe drought stress, and to explore the effect of HA on drought tolerance of oat and its mechanism. The results show that: (1) compared with CK, the contents of superoxide anion (O₂^-·), hydroxyl radical (·OH), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), as well as the activities of superoxide dismutase (SOD) and peroxidase (POD) in oat leaves were significantly increased under severe drought stress, and all of them were significantly decreased after spraying HA. However, the level of active oxygen was still significantly higher than CK (P<0.05). (2) Compared with CK, the activities of catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione peroxidase (GPX) in oat leaves were significantly decreased under severe drought stress, while the total antioxidant capacity (TAOC) was significantly increased after spraying HA, but the activity of each enzyme was still lower than CK (P<0.05). (3) Compared with CK, the grain yield and biological yield of oat were significantly decreased under severe drought stress, and significantly increased after spraying HA, but still significantly lower than CK (P<0.05). In conclusion, spraying HA under severe drought stress can effectively improve the antioxidant enzyme activity of oat leaves, promote renewable antioxidants, enhance the total antioxidant capacity, thereby effectively remove caused severe drought stress accumulation of reactive oxygen species, reduce oxidative damage of severe drought stress on plant cell membrane, eventually relieve severe drought stress to the harm of oats, make up for the loss of grain yield to a certain extent.