Phenotypic characteristics are one of the most valuable parameters for understanding the adaptation of plants to climatic changes. Leymus chinensis, a dominant perennial grass, is widely distributed in the eastern Eurasian Steppe, providing an ideal case study for investigating phenotypic variation across an environmental gradient. In this study, seedlings of L. chinensis were transplanted from the four provenances Zhenlai, Taobei, Da'an, and Jiangjiadiancaochang and grown in a common garden. The aim was to investigate phenotypic plasticity among the four provenances. We measured plant height, leaf length, leaf width, dry weight, water use efficiency (WUE), specific leaf area (SLA), and biomass allocation of plants grown in the common garden. The results indicated that plant height, WUE, SLA, transpiration rates (Tr) and dry weight differed significantly among the four provenances. Taobei provenance plants had the largest plant height, leaf length, and leaf width, which were 72.2 cm, 34.4 cm and 10.34 mm, respectively. Zhenlai provenance plants had the greatest biomass and WUE of 46.05 g and 3.06 μmol CO₂/mmol H₂O and also presented higher fresh and dry weights of 2.88 g and 0.96 g, respectively. There were no significant differences between provenances for net photosynthetic (Pn) and tiller number. These phenotypic and physiological variations found are likely to be a result of long-term environmental adaptation of L. chinensis, which could be conducive to its adaptation to climatic change in the future.