Abstract: Understanding the characteristics of rainy seasons in the Jinsha River was significant for the development of regional water resources, as well as for the prevention and mitigation of flood and drought disasters. Based on daily precipitation data from 1961 to 2023 in the middle-to-lower reaches of the Jinsha River, the onset date, termination date, length, and total precipitation of the rainy seasons were defined, using the established criteria. The spatiotemporal characteristics, changing trends, and atmospheric circulation features during transitional periods were also analyzed. The results showed that, the onset of the rainy seasons had progressed from the northeast towards the west, while the end of the rainy seasons was relatively uniform across the region. The duration of the rainy seasons was longer in the eastern part, compared to the western. The total rainfall during the rainy seasons was higher in the central area than on either side. The onset of the rainy seasons was associated with the northward shift of the South Asian High, coupled with enhanced warm and moist vapor transport from the Bay of Bengal with cold air activity from the west. Conversely, when the wind direction over the Bay of Bengal shifted from southwest to easterly, weakened the warm and moist vapor transport, and westerly winds strengthened over the basin, the rainy seasons came to an end. Climatologically, the rainy seasons lasted for 152 days from May 25th to October 23rd. The average total amount of precipitation during the rainy seasons was 785.2 mm. There were no significant long-term trends in the characteristics of the rainy seasons, however, inter-decadal fluctuations were evident. After the 21st century, there was a delay in the onset date of the rainy seasons. It was also found that the length of the rainy seasons was shortened, and the total precipitation was reduced.