Abstract: Abnormality of summer monsoon circulation in East Asia has special impact on the precipitation in target area, and the low frequency precipitation is an important character of the monsoon. By use of the SCSMEX in situ observational precipitation data and NCEP/NCAR reanalysis data sets, the observed facts related to the abnormality of the precipitation and the related low frequency variability in 1998 are revealed, and the following results are obtained mainly. It is found that the second stage of Baiu come forth in Yangtze River area during the flood season in 1998, 10—20 d low frequency fluctuation exists universally, but 30—50 d one has a regional character. The abnormality of the precipitation in the area of Yangtze River is caused by the abnormity of the location of the west Pacific subtropical high, the abnormity of the persistence blocking high over mid latitude Eurasian land, the abnormity of upper air westerlies jet stream axis, and the second stage of monsoon surges and their interaction. The regional torrential rainfall periods include two stages, the first period is from June 12 to June 26, and the second period begins on July 17 till August 9. The division can pass the significance test for α=0.01. In 1998, the maximum entropy spectrum analysis of the summertime precipitation shows that the preferred period of more than 20 days is of only 4% of the stations during the analysis area(about 136 stations), and the preferred period of less than 10 days is of 31% of the stations, so the primary preferred period is 10—20 days near the East Asia monsoon active region. There are two high percentage areas of mean square error for 10—20 days low frequency oscillation in the target area, and two weeks fluctuation period is an important phenomenon in the flood year, so torrential rainfall cluster occurring time is a main phase of low frequency precipitation. Evidence suggests that the location of the west Pacific sub tropical high has two staggered northwards, and three times rapidly southward withdraw, whereas the location is a little southward in general. Morlet wave period analysis confirms that the sub tropical high ridge line has a low frequency oscillation character, and 10—20 days period occurs in May, and two kinds of LFO period appear from June to August. Meanwhile, the persistence blocking high over mid latitude Eurasian land is strong during 2 torrential rainfall stages, while the cold air can easily move southward, and mid high latitude circulation abnormality looks like i n 1954. Upper level westerly jet rapidly jumps northward near 50°N in early May, and one week late, the location of the jet axis withdraws southward. Since middle July, 200 hPa zonal wind axis has 5—8 latitude distance than normal years. At the same time, two times monsoon surges can be found, the θ_se and southerly have staggered intensities.