Abstract: Meiyu is an important transition period from spring to summer in the Huaihe and Yangtze River Reaches. The traditional methods to diagnose the onset of Meiyu are mainly based on rainy days, temperatures and position of subtropical high, etc. But because of the discontinuous rainy days and variable synoptic background, the traditional methods can't reach consistent results to the onset of Meiyu in many years. A new method is introduced to diagnose the onset of Meiyu using moisture. Characteristics of moisture variation during Meiyu season in recent four years are analyzed based on the precipitable water vapor data from GPS Meteorological Network (thereafter as GPS/PWV) in the Yangtze River Delta. It is founded that the GPS/PWV changes significantly on the beginning of Meiyu, which shows a seasonal change of moisture. The following three indexes of GPS/PWV are concluded to diagnose the onset of Meiyu:Everyday average and five-day average of GPS/PWV ascend to more than 40 mm; five-day average of GPS/PWV is stably under 40 mm before the onset of Meiyu, occasionally over 40 mm, after the onset of Meiyu, five-day average is stably over 40 mm, occasionally under 40 mm; the difference of five-day average shows a peak more than 20 mm at about the onset of Meiyu.If the three requirements are met by the GPS/PWV variation, it can be considered as the onset of Meiyu. This method is tested by PWV from 1980 to 2000 based on radiosonde sounding data. Results show that the onset days of Meiyu diagnosed by new method are more reasonable than those by traditional methods; the seasonal transformation can be better illustrated by the moisture data. This method is also successfully applied to diagnose the onset of Meiyu in 2006. The onset day of Meiyu in 2006 over the lower reaches of the Yangtze River issued by different observatories is different, such as 13—14 and 22 of June. Through analysis of GPS/PWV data from June to July, June 22 is more reasonable as the onset day of Meiyu in 2006.In addition, analysis shows the increasing of water vapor is directly led to by north-moving of subtropical high, which is advantageous to the establishment of Meiyu front.