The Qinghai-Tibet Plateau (QTP) has experienced an overall increase in humidity over recent decades, but the changes in aridity and humidity within the transitional climate zone, as well as the underlying factors driving these changes, remain unclear. We present a framework to quantify the spatiotemporal variations transitional zone indicated by 400 mm isohyets and its driving factors on the QTP. The results show that the 400 mm isohyets has moved an average of 87.85 km to the north-west direction, moving an average 1.6 km per year, and a maximum distance traveled of 261 km (northernmost orientation) during the past four decades. Over time, the variation in isohyets exhibited distinct piecewise characteristics and obvious spatial differences. The central section (95◦E–101◦E) remained relatively stable, whereas the eastern section (101◦E–104◦E) and western section(91◦E–95◦E) experienced violent fluctuations. The east–west boundary of the spatiotemporal movement of the 400 mm isohyets was utilized to delineate the eastern humid zone, transitional zone, and western arid zone. The transition zone interannual precipitation has experiences more significant interdecadal oscillations between dry and wet climates, while the western region has experienced the opposite. The East Asian summer monsoon (EASM) exhibited a positive correlation with precipitation, increased precipitation is the main factor driving the westward and northward movement of the 400 mm isohyets. The vegetation response to precipitation exhibited greater variability in the transitional zone. This study provides a novel reference for quantitatively comprehending the aridity and humidity of climatic conditions, as well as the driving mechanisms behind regional climate change in the QTP.