Phenology plays an important role in indicating climate change. However, current researches mainly focus on the relationship between phenological changes and environmental factors. There is a lack of research on ecophysiological mechanisms which cause phenological changes. Based on a simulation experiment on Stipa krylovii in the Inner Mongolia Autonomous Region under different treatments of infrared radiation heating and water control, the photosynthetic ecophysiological mechanism of Stipa krylovii in response to climate change was examined. (1) The green-up date and heading date were advanced (2.8 d & 7.8 d) under increasing temperature treatment, while the wilting date was delayed (6.8 d). The interaction of temperature and precipitation mainly affected the heading date. Compared with Stipa krylovii under increasing temperature and decreasing precipitation treatment (climate warming and drying), those under increasing temperature and increasing precipitation treatment (climate warming and wetting) were 7.4 days earlier. (2) The net photosynthetic rate, stomatal conductance and transpiration rate of Stipa krylovii were significantly higher at green-up stage and heading stage under the increasing temperature and increasing precipitation treatment, while water use efficiency was lower than those under increasing temperature and decreasing precipitation treatment. There was no significant difference in photosynthetic ecophysiological characteristics between increasing temperature and precipitation treatment and increasing temperature and decreasing precipitation treatment at wilting stage. (3) Net photosynthetic rate was the decision-making factor affecting the change of plant phenology. Temperature was the limiting factor affecting green-up date and wilting date, while precipitation was the limiting factor affecting heading date. The results showed that the phenology of Stipa krylovii was closely related to both photosynthetic ecophysiological characteristics and environmental factors. This study could provide a theoretical support for the development of plant phenological model and the understanding of phenological mechanism.