Hydrogen (H₂), an endogenous gaseous molecule, plays a significant role in plant development and stress responses. Here, we investigated the effects of hydrogen-rich water (HRW) pretreatment on 64 parameters, including germination rate, growth indexes, sugar mobilization, reactive oxygen homeostasis, enzymatic and non-enzymatic antioxidants defense system under drought of barley (Hordeum vulgare L.). The results showed that exogenous H₂ supplies differentially attenuated the damage of germination and seedling establishment by drought. Compared to samples treated with drought stress alone, HRW pretreatment promoted germination rate and seedling morphological parameters (length and number of root, length of shoot and coleoptile). Further research exhibited HRW could elevate reducing sugar content, α-amylase activity, and β-amylase activity in the seed, significantly decreased the membrane injury index, superoxide radical (O₂⁻) level and hydrogen peroxide (H₂O₂) level, increased the activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR), both in root and shoot. What is more, exogenous H₂ also promoted ascorbate (AsA) content and glutathione (GSH) content, reduced dehydroascorbic acid (DHA) content and glutathione disulfide (GSSG) content to different degrees, which led to AsA/DHA ratio and GSH/GSSG ratio increase. The above results combined with correlation analysis and principal component analysis suggested that HRW (especially 25% HRW) pretreatment could mitigate drought-induced damage enabled by activating sugar metabolism through up-regulating amylase activity, reestablishing redox balance through regulating the ascorbate-glutathione (ASA-GSH) cycle.