The seasonal vegetation greening in the Three River Source Region（TRSR）has a profound impact on the ecological environment and water resource security. In this study，the moisture drivers of seasonal vegetation greening in the TRSR and their responses to climate change were investigated using multi-source data from 2003 to 2021，through the application of trend analysis，correlation analysis and partial information decomposition （PID）analysis. The results showed that（1）the linear trend of the Leaf Area Index（LAI）generally increased in spring，summer and autumn，although the environmental conditions varied significantly between seasons. Linear trends in precipitation，soil moisture（SM）and snow cover（SC）also increased in spring and autumn，with in‐ significant changes in temperature. In summer，linear trends of temperature were slightly higher，with slight de‐ creases in precipitation and SM，as well as insignificant changes in SC.（2）Effects of moisture drivers on LAI： correlation analyses indicated that LAI was significantly positively correlated with SM in spring and summer， but not in autumn. The correlation between LAI and SC was weak in all seasons. By introducing the PID analysis method，the nonlinear and synergistic effects of SM and SC on LAI were effectively revealed. The independent information contribution of SC to LAI changes was higher in spring and autumn，making it the main moisture driver in these seasons，while SM contributed more in summer. At the same time，the synergistic effects of SM and SC played an important role in the changes of LAI in all seasons，with the synergistic information contribution exceeding 30% in all seasons.（3）Response of moisture drivers to climate change：correlation analyses showed that SM was significantly positively correlated with precipitation in all seasons and significantly negatively correlated with temperature in spring；SC was significantly positively correlated with precipitation in all sea‐ sons and significantly negatively correlated with temperature in both spring and autumn. PID analyses also indicated that precipitation was the main meteorological factor influencing changes in SM and SC across the three seasons，with a higher independent contribution than temperature. However，the synergistic effects of temperature and precipitation on SM and SC in all seasons should not be overlooked.