In terms of the NCEP/NCAR daily reanalysis data,an index based on the vorticity value in the key area is defined to describe the longitudinal position of the western Pacific subtropical high(SH).An investigation is carried out of variation features of the longitudinal position of the SH in June.Results show that the index avoids the interdecadal change of the westward stretch of the SH resulted from the yearly geopotential height rising,and well reflects the character of large-scale circulation field.In the wester subtropical high year,the equatorial westerly and the cross-equatorial flow weaken,the South China Sea summer monsoon is weaker.The southerly enhances over the south of Changjiang River and South China areas,then the negative vorticity develops in the east of these areas leading to the westward stretch of the SH.The anomalous southwesterly in the north side of the SH dominates over the Changjiang River Valley in favor of the maintaining of rainbelt,bringing about more rainfall(flood).The wavelet analysis suggests that the strongest periodical signal of the index in June is 2,4,and 8-year.