Abstract. Precipitation over Southwest China (SWC) significantly decreased during 1979–2013. The months from July to September (JAS) contributed the most to the decrease in precipitation. By tracing moisture sources of JAS precipitation over the SWC region, it is found that most moisture originates in regions from the northern Indian Ocean to SWC and from South China Sea to SWC. The major moisture contributing area is divided into an extended west region, SWC, and an extended east region. The extended west region is mainly influenced by the South Asian summer monsoon (SASM) and the westerlies, while the extended east region is mainly influenced by the East Asian summer monsoon (EASM). The extended west, SWC, and extended east regions contribute 48.2, 15.5, and 24.5 % of the moisture for the SWC precipitation, respectively. Moisture supply from the extended west region decreased at a rate of −7.9 mm month−1 decade−1, whereas that from the extended east increased at a rate of 1.4 mm month−1 decade−1, resulting in an overall decrease in moisture supply. Further analysis reveals that the decline of JAS precipitation is mainly caused by change in the seasonal-mean component rather than the transient component of the moisture transport over the SWC region. In addition, the dynamic processes (i.e., changes in wind) rather than the thermodynamic processes (i.e., changes in specific humidity) are dominant in affecting the seasonal-mean moisture transport. A prevailing easterly anomaly of moisture transport that weakened moisture supply from the Indian Ocean is to a large extent responsible for the precipitation decrease over the SWC region.