Deschampsia caespitosa has the characteristics of cold resistance and pruning resistance. They can grow not only in arid environments such as grassland, but also in wet habitats such as river beach and swamp. D. caespitosa has high seed yield and high germination rate. It is one of the ideal species for vegetation restoration in the alpine marshes. Water is one of the important abiotic factors limiting many aspects of plant growth and development, including survival, metabolism, and geographical distributions. Water deficit is the most common stress factor during growth and development. Plants have evolved an effective antioxidant system to remove excess reactive oxygen species to protect them from damage. Proline (Pro), a widely distributed osmotic regulator, plays an important role in preventing plants from damage under water stress. In order to study the physiological mechanism of D. caespitosa growth under drought and flood stress, we examined Pro accumulation and metabolism in the leaves of D. caespitosa to understand how this herbage plant tolerates drought and waterlogging stress in the alpine marshes. We analyzed Pro accumulation and changes of the substrate, intermediate products and enzyme activities in the Pro metabolism under drought and waterlogging stress. We found that the Pro content was increased in the leaves of D. caespitosa under drought stress and waterlogging stress. Pro levels were increased over the period of treatment but were leveled off or decreased after treatment for 21 days. Glutamate (Glu) and ornithine (Orn) levels were decreased in the leaves of D. caespitosa under drought stress and waterlogging stress, while Glu was changed more obviously. Glu and Orn were involved in Pro metabolism that leading to Pro production. The levels of glutamic-γ-semialdehyde(GSA)and Δ¹-pyrroline-5-carboxylate(P5C), intermediates of the Pro metabolism, changed under water stress. The activities of Δ¹-pyrroline-5-carboxylic acid synthetase (P5CS), ornithine aminotransferase (δ-OAT), and Δ¹-pyrroline-5-carboxylic acid reductase (P5CR) were increased, while the activity of proline dehydrogenase (ProDH) was reduced. Osmotic stress was alleviated in the leaves of D. caespitosa by increasing Pro content. Pro increase was caused by the active Pro accumulation and the interaction of the Glu pathway and Orn pathway. Pro accumulation was enhanced by Pro anabolism and inhibited by Pro catabolism. D. caespitosa was died after the severe drought treatment for 21 days and severe waterlogging treatment for 28 days. Our results are helpful for future in-depth study of plant co-tolerance to drought and flood stress in alpine marshes as well as for the restoration of the degraded alpine marshes with herbage plants.