The oat variety ‘Yanke 2’ was used as the experimental material, and potted plants were used in normal water supply (75% field water holding capacity), moderate drought stress (60% field water holding capacity) and severe drought stress (45% field water holding capacity). Spraying humic acid (HA) and the same amount of water (CK) under 3 water conditions, we determined the content of nonstructural carbohydrates (NSC) in oat leaves, related enzyme activities and grain yield. The effect of HA on the drought tolerance of oats and its mechanism were preliminarily discussed on the changes of nonstructural carbohydrate metabolism in oat leaves. The results showed that： (1) with the decrease of soil moisture content, the sucrose and starch contents in oat leaves gradually decreased significantly, and the activities of sucrose synthase (SS) and sucrose phosphate synthase (SPS) decreased significantly, while the activities of acid invertase (SAI) and amylolytic enzyme (αGC) increased significantly. (2) The contents of total soluble sugar and reducing sugar in leaves showed a trend of increasing first and then decreasing, leading to a significant decrease in grain yield, and the heavier the drought stress, the greater the decrease. (3) Foliar spraying of HA can increase nonstructural carbohydrate content in oat leaves under moderate and severe drought stress to varying degrees, regulate related enzyme activities, significantly increase grain yield, and have a better effect under severe stress. Studies have found that humic acid can respond to drought stress by regulating the metabolism of oat leaf NSC, reducing leaf cell osmotic potential, effectively alleviating the damage caused by drought stress, and enhancing plant drought tolerance.