Using Wuhan twice daily radiosounding data from June to July in 2004, the preci -pitable water, specific humidity and relative humidity are calculated. By contras ting and analyzing the characteristics of the vertical distribution and the dail y variation of those vapor parameters, the evolution characteristics of the vapo r over Wuhan in Meiyu period in 2004 and its relationship with heavy rainfall ar e discussed. The result shows that the vapor content in the air over Wuhan decreases with the height and 90 percent of vapor concentrates below 700hPa.The vapor content in July is more than that in June. The specific humidity increase in the whole layer or in the middleand upperlevel can lead to the evident in crease of the whole layer relative humidity over Wuhan, and the increasing degree inc reases with the height. In the course of analyzing and forecasting heavy rain, i t is better to analyze the vapor evolution in the middleand upperlevel than that in the lowlevel. They are all very meaningful indicators for heavy rain f orecast to analyze the total layer vapor evolution combining the total layer mea n relative humidity with precipitable water, and to analyze the middle and upp er level vapor evolution combining average specific humi-dity with relative hu midity of the middle and upper level.