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Climate transition at different scales complicates the dynamics of catchment water cycle. Examination of meteorological pattern shift enables us to have a deeper insight of the mechanism in hydrological extremes. Lake Hulun, the largest freshwater lake in Northern China, plays an important role in maintaining the regional ecological sustainability. The abrupt decrease of water level in the lake after 2000 has resulted in serious impacts on the lake's ecosystem. This paper presents a systematic analysis by linking meteorological dynamics with hydrological extremes based on the long-term precipitation, river discharge, and evapotranspiration (ET) data in the Lake Hulun watershed, which is a transition zone from desert to wetland and to permafrost along the Mongolia–China borderline. The results indicate that signals of large-scale precipitation variations can be well identified by using the Empirical Orthogonal Function (EOF). Spatially, the precipitation in the Lake Hulun watershed is dominated by two leading EOF patterns at the basin scale. The two leading patterns present complementary features of the high value centers in space. The amplitudes evolution of the leading patterns of EOF reveal that the redistribution of precipitation that occurred more in the middle flat plain and less in the mountain regions of the watershed triggers a big effect on the increasing ET and weakening runoff production due to the spatial heterogeneity in hydrological processes. Such effects are echoed by the trend patterns in precipitation and ET. The mechanism of the precipitation pattern shift is also confirmed by the large scale meteorological variations in geopotential and water vapor transport. Therefore, the change in the leading patterns of precipitation and the effects in ET triggered by the pattern shift in precipitation are the major causes of decrease in watershed runoff and the hydrological extremes in the terminal lake. [ABSTRACT FROM AUTHOR]