Abstract: The dominant mode of anomalous precipitation in May over Southwest China and its association with the Arabian Sea monsoon are investigated in this study using the 115 observation stations and Japanese 55-year reanalysis for the period of 1960–2017. The result showed approximately consistent variabilities of the leading mode of May precipitation over Southwest China, exhibiting a close relationship with the Arabian Sea monsoon. However, around the late 1970s, the relationship experienced an interdecadal change. In 1960–1976, the anomalous atmospheric circulations and water vapor transportation associated with the Arabian Sea monsoon were mainly over the Arabian Sea to the Bay of Bengal, showing a weak influence on the May precipitation over Southwest China. However, in 1981–2017, the anomalous Arabian Sea monsoon was related to large-scale atmospheric anomalies from the northern Indian Ocean to the South China Sea, leading to anomalous water vapor and vertical motion over Southwest China. Therefore, the Arabian Sea monsoon can significantly influence May precipitation over Southwest China during 1981–2017. Further analysis indicated that the change in the relationship between the Arabian Sea monsoon and May precipitation over Southwest China could be related to the change in decadal variability of the Arabian Sea monsoon. Over the period before the late 1970s, the Arabian Sea monsoon’s variability was relatively weaker, and its related atmospheric circulation anomalies were also weaker. Whereas, after the late 1970s, the Arabian Sea monsoon’ s variability became stronger, and the related atmospheric circulation anomalies extended more eastward, covering Southwest China. Consequently, the Arabian Sea monsoon can significantly influence the May precipitation over Southwest China after the late 1970s. This result indicated the important role of monsoon variability in precipitation.