Constructed grasslands are primary restoration measures in areas with degraded natural grasslands. Grass–legume mixtures are chosen to obtain high production and forage quality; however, the photosynthetic and other traits of such mixtures are not well understood. In this study, we evaluated the effects of grass–legume mixtures on the growth and photosynthetic capacity of three forage crops over two growing seasons. Bromus inermis and Elymus nutans were grown as monocultures or in mixtures with Medicago sativa. We analysed forage yields, quality, gas exchange and chlorophyll fluorescence parameters. The grass–legume mixtures improved the forage yield, root : shoot ratio, and contents of crude protein and lignin. Compared with the monoculture, grasses in the mixtures had higher net photosynthesis, water-use efficiency (WUE), and leaf nitrogen (N) content, but lower carbon : N ratio, and distributed more absorbed light to photosynthetic electron transport and thermal dissipation. In the mixture, B. inermis had a higher light-saturation point, indicating high light-use efficiency. Elymus nutans had a lower light-compensation point and dark respiration rate, suggesting good shade tolerance. However, water deficits decreased biomass and photosynthetic capacity in the E. nutans–M. sativa mixture, suggesting that E. nutans was sensitive to soil moisture. The B. inermis–M. sativa mixture had greater and more consistent biomass and WUE.The grass—legume mixture is an important way to construct grasslands. We analysed forage yields, quality, gas exchange and chlorophyll fluorescence in the mixtures, finding that grass—legume mixtures improved forage production and photosynthetic capacity. The study provides a scientific basis for establishment of constructed grasslands with high yield and quality.