Abstract: The extensive marsh wetlands in Northeast China’s Songnen Plain play important roles in biodiversity protection and regional carbon cycle. Revealing the phenological variations in marsh vegetation and climatic change influences is indispensable for assessing carbon sequestration and maintaining ecological balance in the Songnen Plain. Using climate and normalized difference vegetation index (NDVI) datasets in 2001–2020, this study investigated the spatiotemporal changes of the length (LOS), end (EOS) and start (SOS) of the growing season, as well as influences of climate change on marsh vegetation phenology. Our results revealed a significant (P < 0.05), widespread extension of the growing season during 2001–2020, with the LOS increasing by 0.38 d/yr. The primary driver behind this trend was the significant rate of SOS advancement (P < 0.05), which reached 0.34 d/yr. Increased precipitation in May significantly (P < 0.05) advanced SOS, whereas increased precipitation in October significantly shortened LOS by advancing EOS in the marshes of the Songnen Plain. Warming spring and winter temperatures advanced SOS, and warming autumn temperature delayed EOS. Our results indicate a previously unreported asymmetric effect of nighttime versus daytime warming on marsh vegetation phenology within the Songnen Plain. Increased daytime temperature had a stronger delaying influence on EOS than that observed for increased nighttime temperature from October to November. This study reveals the differential impacts of monthly temperature and precipitation variations on marsh vegetation phenology, and emphasizes the necessity of incorporating asymmetric diurnal warming in phenological modeling. These findings could deepen our understanding of phenological responses to climate change in arid and semi-arid wetland ecosystems.