In August 2021, extremely unusual precipitation in the eastern part of Southwest China reached the second highest next to 1998 since 1961. Based on daily precipitation and monthly ERA5 reanalysis data during 1961-2021, the main water vapour transport conditions and water vapour sources, along with the abnormal atmospheric circulation characteristics for anomalous more than normal precipitation in August 2021 over the eastern part of SW China, are analyzed. The results show that the net inflow of water vapour in the eastern part of southwest China in August 2021 is mainly from the mid-lower troposphere, with the largest contribution from the lowest layer. In August 2021, the water vapour transport pathway is the most unique one, different from the main trajectory and contribution from the southern pathway. In August 2021, the abnormal precipitation in the eastern part of southwest China is closely related to the abnormal atmospheric circulation, mainly reflected in the 500 hPa with blocking system over the Ural Mountains and the sea of Okhotsk resistance, and the abnormally active mid-latitude low system (i.e. Northeast cold vortex), the weak Indian low pressure and the western subtropical high abnormal westward extension and strong intensity. The favourable water vapour transport guided by the atmospheric circulation system results in more abnormal precipitation in the eastern part of Southwest China. Persistent warming of the Indian Ocean is likely an important external forcing factor maintaining the continuously intensified Pacific subtropical high in August 2021, while the abnormal strength of the Okhotsk high in the same period could be related to the abnormal warming of the Northwest Pacific SST. As a result, they play a positive role in the excessive precipitation in the Southwest China region.