Based on the ERAInterim reanalysis data, an intensity index of the South Branch Trough (SBT) is defined by the result of the objective identification method of the SBT. The relationships between the interannual variations of the SBT intensity and the precipitation, atmospheric circulation, and water vapour transport during 1980-2019 are investigated in this study. The results show: (1) The intensity index of the SBT mainly characterizes interannual variations with the quasi4year cycle. (2) The sea surface temperature in the central equatorial Pacific is an important factor affecting the interannual variations of the SBT, which has a significant negative correlation with the intensity of the SBT, especially the SST over the Nino 3 region. (3) When the intensity of the SBT is stronger, the precipitation from southern Yunnan to South China is more than normal, especially in Guangdong, southern Hunan, southern Jiangxi, and southern Fujian, which is 30 mm more than normal. When the intensity of SBT is weaker, the precipitation in Southwest China to South China is less than normal. In particular, the precipitation in central Guangdong and Guangxi is over 30 mm less than normal. (4) There are two water vapour transport paths with strong intensity of SBT. One is the water vapour conveying belt with anomalously strong westerly wind extending from the north of the equator to 10°N and 60°E to the Philippines. The other branch is located near 20°N, and the southwest airflow in front of the SBT transports the water vapour from the Bay of Bengal area to China, which is the main water vapour channel of Southwest and South China in winter.