Abstract: To clarify the alleviating effect of exogenous uniconazole on potatoes under drought stress, the drought-tolerant variety ‘KX1’ and the drought-sensitive variety ‘Atlantic’ were used as experimental materials. Drought treatment (maintaining 45% relative soil water content) and spraying uniconazole (40 mg·L⁻¹) were carried out at the tuber expansion stage (second inflorescence flowering). Three treatments were established for each variety: control (CK), drought stress (D), and drought stress + uniconazole (D+S). The chlorophyll content, photosynthetic parameters, membrane lipid peroxidation (malonaldehyde, MDA), reactive oxygen species (ROS), and antioxidant enzyme activity in the antioxidant defense system, as well as ascorbate aicd–glutathione (AsA-GSH) cycle products and substrate content in potato leaves under exogenous uniconazole and drought stress were measured. The results showed that drought stress reduced the chlorophyll content of the two varieties leaves, and significantly inhibited net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), but significantly increased MDA, H₂O₂ content, and O₂⁻ production rate. On the 15th day of drought stress, MDA and H₂O₂ contents, and O₂⁻ production rate in D treatment of ‘KX1’ and ‘Atlantic’ leaves were significantly increased by 29.2% and 43.6%, 55.6% and 73.1%, 84.3% and 110.6%, respectively, compared to CK; the antioxidant enzyme activity and non-enzymatic antioxidant content of both varieties increased. The drought-tolerant variety ‘KX1’ was less affected by drought than the drought-sensitive variety ‘Atlantic’. Exogenous uniconazole significantly increased the chlorophyll content and photosynthetic parameters of potato leaves, reduced the MDA content, O₂⁻ production rate, and H₂O₂ content in leaves of ‘KX1’ and ‘Atlantic’, and increased the activity of antioxidant enzymes in the antioxidant defense system. In addition, uniconazole further increased the contents of ascorbic acid (AsA), dehydroascorbate (DHA), glutathione (GSH), and oxidized glutathione (GSSG) in the AsA-GSH cycle of the two potato varieties under drought stress and increased the ratio of AsA/DHA and GSH/GSSG. In conclusion, exogenous uniconazole can promote leaf photosynthesis to a certain extent and improve the antioxidant defense capacity to alleviate the damage caused by drought stress. These results provide a theoretical basis for drought-tolerant cultivation.