Using ERA-Interim and APHRO_MA data, the spring precipitation decadal trend in the western Tibetan Plateau during 1979-2007 and its possible causes were analyzed. The results indicate that the change in spring precipitation in the western Qinghai-Tibet Plateau presented a significant decreasing trend, which is clearly related to both the convergence rising in the southwestern Tibetan Plateau and water vapor transport from the northern Arabian Sea. A significant convergence anomaly was found to be generated when the negative high-level (500 hPa) geopotential height field and negative low-level convergence (850 hPa) were occupied, which provided the dynamic conditions for spring precipitation in the southwestern Tibetan Plateau. Moreover, the location of the negative high-level geopotential height field anomaly was consistent with the cyclonic anomaly center in the wind field, while that of the low-level convergence was relatively weak. The spring precipitation anomaly was accompanied by both specific humidity anomalies at the high- and low-level geopotential height fields in the northern Arabian Sea; however, the positive specific humidity anomalies of the lower layers were more significant. The water vapor transport in the northern Arabian Sea accounted for 55.3% of the total spring precipitation change in the southwestern Tibetan Plateau. The study shows that the decreasing trend of spring precipitation in the southwestern Tibetan Plateau from 1979 to 2007 was mainly caused by the decreasing water vapor transport from the lower layer of the northern Arabian Sea to the Indian subcontinent and the weakening high-level convergence of the southern Tibetan Plateau. The consistency of trend in spring precipitation in the western Tibetan Plateau and water vapor changes in related areas can provide guidance to analyze the climate change in the Tibetan Plateau.