Ultraviolet-B radiation (UV-B) has a dual effect on plants, enhancing growth under low doses but causing oxidative damage, irregular photosynthesis, and abnormal growth at high doses, negatively impacting lignin biosynthesis, stem strength, and yield. Abscisic acid (ABA) mitigates these effects by promoting photosynthesis, regulating growth, and maintaining hormonal flux. Limited attention has been given to the role of ABA in stress mitigation and yield improvement for high-altitude crops like qingke (highland barley; Hordeum vulgare L. var. nudum). This study investigates the effects of UV-B radiation and ABA on the physiological, biochemical, transcriptomic, metabolomic, and hormonal responses of qingke. Low-dose UV-B (LUVB; 5–6 kJ m−2 d−1) increased plant height by 51.3% but reduced stem diameter by 31.6%. ABA supplementation (70 μM) under UV-B (ABA-UVB) moderated height (20.7% increase) and improved stem diameter by 8.1%. The ABA under UV-B increased tillers (57.1%), spikes (66.5%), and yield (26.2%). At the molecular level, this treatment upregulated lignin biosynthesis genes transcripts (PAL, 4CL, CAD, POD) and enhanced enzymatic activities by 69.0% to 150.1%. Metabolomic analysis revealed elevated lignin precursors, including p-coumaryl alcohol and sinapyl alcohol. Hormonal profiling showed ABA restored Indole-3-acetic acid (IAA) and gibberellic acid (GA) levels, elevated trans-zeatin (tZ), and maintained hormonal balance. Nutrient analysis revealed higher leaf magnesium (Mg) by 28% and calcium (Ca) by 40%, along with increased root manganese (Mn) and copper (Cu) levels under UVB-ABA, which enhanced stress adaptation. These physiological, transcriptomic, and metabolomic changes collectively strengthened stems, improved lodging resistance, and enhanced yield suggesting the potential of ABA for mitigating UV-B stress in high-altitude crops.