Although partial root-zone drying irrigation (PRD) has been proven to be applicable to alfalfa production, the physiological mechanism of how PRD improves water use efficiency without biomass loss in alfalfa has not been well documented. We designed a field experiment with two irrigation modes (PRD and conventional furrow irrigation (CFI)) and four irrigation volumes (70%-I₁, 85%-I₂, 100%-I₃, and 115%-I₄ of alfalfa water requirement (ET)) to determine the effect of PRD on gas exchange and antioxidant enzymatic activities. Our study showed that PRD did not markedly affect the biomass but reduced the leaf area of alfalfa plants at the first and second harvest times. Whether for the first or second harvest time, PRD had no significant effect on net photosynthetic rate (Pn) but decreased the stomatal conductance (Gs) and transpiration rate (Tr), resulting in higher leaf water use efficiency (LWUE). PRD significantly increased the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, as well as the soluble sugar, proline, and malondialdehyde (MDA) concentrations, and decreased the soluble protein concentration at the first and the second harvest times. These results suggest that PRD could improve photosynthetic characteristics, increase oxidation resistance, and regulate osmotic adjustment. This would conserve irrigation water volumes and maintain a relatively steady biomass of alfalfa plants. Our findings present physiological evidence that PRD can be successfully applied to alfalfa production.