South China is an important water vapor transport channel. Studying the water vapor budget in South China is essential for predicting precipitation processes in South China and its neighboring areas, and comprehending the impact of atmospheric circulation changes on weather and climate in China. Based on ERA5 data, the variation trend of water vapor budget and its correlation with precipitation in South China during the recent 40 years (1983-2022) are analyzed. The results show that water vapor in South China is mainly input through its the southern and western boundaries, and output through the northern and eastern boundaries, with the net budget being negative. The water vapor budget shows a declining trend, with a significant decrease in the output from the eastern boundary. In the four seasons, the input and output water vapor budget declines most obviously in spring. Water vapor enters into the South China Region mainly from the Indian Ocean and the Bay of Bengal in the southwest direction and western Pacific in the southwest direction. Water vapor in the middle and lower layers (700 hPa) is primarily transported from the Indian Ocean and the Bay of Bengal, while water vapor in the lower layer (925 hPa) mainly originates from western Pacific. In the recent 40 years, water vapor transport from the Indian Ocean and the Bay of Bengal and western Pacific has been weakening, causing the changes in water vapor transport from the direction of northeast to southwest in South China. Water vapor transport is positively correlated to precipitation in most areas of Guangxi and Guangdong, with correlation coefficient >0.6, of which the strengthening of southwestly water vapor transport is the key reason of precipitation. In addition, there is a trend of wetting in South China, with a 2.32% increase in precipitable water vapor (PWV) during the recent 40 years, which is the result of decrease in total water vapor outflow in the region than that of total inflow. The results of this study can provide a reference for further understanding of water vapor budget changes and abnormal precipitation events in South China.