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Climatic instability, especially on decadal to multi-decadal time scales, has a profound impact on societal development and human welfare in semi-arid and arid regions. Previous work suggested that arid northwestern China was characterized by a dry Medieval Warm Period (MWP, from ca. 1000 to 1300AD) and a wet Little Ice Age (LIA, from ca. 1400 to 1850AD), but there is a lack of studies addressing the issue of climatic variability during the last millennium. Here, we present results from a diverse range of proxy records from northwestern China with the aim of evaluating the time variation of hydroclimatic variability during the MWP and LIA. Within the context of an overall cold climate, we detected a pattern of higher moisture variability during the LIA compared to the MWP. A comparison of the proxy records of climatic instability during the LIA from northwestern China with the major modes of SST and atmospheric variability, which affect moisture/precipitation in Asia, suggests that the instability of the North Atlantic Oscillation (NAO) from middle- to high-latitude regions during the LIA was a major driving factor. The westerlies may play an important role in linking the unstable climatic conditions observed in the above-mentioned regions. Our results support the concept that climatic instability was above the norm during cold periods, in contrast with present observations that demonstrate the occurrence of increasingly extreme climatic events with ongoing global warming.