Based on the daily precipitation data of 60 stations in South China and NCEP reanalysis data,the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT_4.9) are used to analyze the difference of water vapor transport trajectories of frontal and monsoon precipitation in the pre-flood season in South China,main water vapor sources and precipitation contribution rates of water vapor in main sources during different Pacific Decadal Oscillation (PDO) phases from 1960 to 2012.Results show that:(1)In the frontal precipitation stage,there is more water vapor in the Western Pacific-the South China Sea-the Bay of Bengal during the positive PDO phase.In the summer monsoon precipitation stage,the water vapor in the northern Indian Ocean-the Bay of Bengal-the South China Sea is more during the positive PDO phase.(2)In the frontal precipitation stage,the water vapor mainly comes from the Northwest Pacific and the South China Sea.During the positive PDO phase,the subtropical high is located to the north,which makes the water vapor transport path in the Western Pacific to the north and transports more water vapor to South China.It is conducive to the formation of monsoon precipitation in South China.There is a significant positive correlation between precipitation and PDO.(3)In the monsoon precipitation stage,during the positive PDO phase,although the water vapor content in the whole layer from the northern Indian Ocean-the Bay of Bengal-the South China Sea is large,it is not completely transported to South China,so the effective monsoon precipitation is less,and there is a significant negative correlation between precipitation and PDO.