In the context of global warming and climate change, significant changes in precipitation have occurred in the drylands of East Asia, where water vapor transport has a significant impact on precipitation anomalies. Recent studies on water vapor sources in the drylands of East Asia have been reviewed and analyzed, focusing on external water vapor transport sources, seasonal differences, and internal evapotranspiration variations. Future research directions are discussed and predicted. Existing studies show that water vapor from the Bay of Bengal–Indian Ocean, the South China Sea, the western Pacific Ocean, and the Eurasian continent are transported to the semi-arid regions of East Asia by the South Asian, South China Sea, and subtropical monsoons, and the mid-latitude westerly wind belt; the water vapor from the South China Sea and the western Pacific Ocean dominates during summer and depends on the water vapor content carried by the westerly winds during winter. Since 1979, the annual precipitation recirculation rate in semi-arid regions of East Asia has been increasing, and the recirculation rate in summer is higher than that in winter on a seasonal scale. The sources and paths that dominate water vapor transport in the drylands of East Asia, particularly summer water vapor transport, need to be further verified. Quantifying the relative contributions of the external water vapor input and internal water vapor evaporation may become a future research topic. Furthermore, the relationship between global changes and changes in precipitation water vapor sources must be analyzed in depth.