The environmental factors, growth rate, chlorophyll content, and photosynthetic parameters in Leymus mollis (Trin.) leaves were measured during the day without wind in the fall and summer, and with high winds in the fall, in the natural environment to understand the physiological adaptation mechanisms of L. mollis to different wind speeds. The results showed that high winds, lower temperatures, and higher humidity were observed on the windward slopes, where L. mollis had higher chlorophyll contents in leaves and shorter plants. However, higher temperatures, drier soils, and poor airflow were observed on the leeward slopes, where L. mollis had lower chlorophyll contents in leaves and taller plants. There was obvious environmental heterogeneity between windward and leeward slopes, and enhanced morphological plasticity. During days of no wind in the fall and summer, L mollis on both slopes showed a midday depression of photosynthesis, but L. mollis on the windward slopes had markedly higher stomatal conductance(Gs), transpiration rate(Tr), and net photosynthetic rate(Pn)than that of the leeward slopes. However, during days of high wind in the fall, L. mollis on both slopes had no midday photosynthetic depression, and L. mollis on the leeward slopes had markedly higher Gs, Tr, and Pn than that on the windward slopes. On the same slopes, during the days of high wind, L. mollis on the windward slopes had lower Pn, Tr, and Gs than that on days with no wind, but higher Pn, Tr, Gs by 126, 66.3, and 134% on the leeward slopes than that on days with no wind. High winds resulted in lower temperatures, reduced stomatal aperture, and leaves on the windward side with reduced Pn. High winds resulted in increased air flow, and reduced temperatures on the leeward slopes, which increased L. mollis stomatal opening, but had no midday photosynthetic depression and higher Pn. L. mollis on both slopes showed significant photosynthetic plasticity to adapt to different wind speeds, enabling it to take advantage of the environmental resources and increase photosynthesis and dry matter accumulation for its sustained survival. Morphological and photosynthetic plasticity of L. mollis is an important physiological regulation mechanism to enhance survival, growth, and population expansion under high winds, which has an important application value for the breeding of crops, grasses, and trees with higher tolerance to wind in the future.