An ultraviolet‐B (UV‐B) exclusion experiment was established in native Tibetan alpine grasslands to investigate the possible effects of ambient UV‐B on ecophysiological characteristics and forage quality. Selective filters were used to reduce UV‐B radiation in the field. During one growing season, specific leaf area (SLA) and net photosynthetic capacity (Pn) were measured on Elymus nutans Griseb. Total flavonoid contents, neutral detergent fiber (NDF), acid detergent fiber (ADF) acid detergent lignin (ADL) and crude protein (CP) were determined from a late season harvest. Compared with the reduced UV‐B treatment, the plants in ambient UV‐B were found to have higher total flavonoid contents, ADL and CP, but lower SLA and Pn. Our results indicated the current level of UV‐B in the Tibetan grasslands is a significant stress factor for E. nutans growth and has differential effects on forage components.