The day-to-day change indexes of the western Pacific subtropical high (STH) at 500hPa from June to July of 1998 and 2001 over the Changjiang River valley are calculated. The atmospheric circulation field of the northern Hemisphere and the daily variation features of STH in the Changjiang River valley during plum rains season are comparatively analyzed. The result shows that the flood/drought in the Changjiang River valley during plum rains season is closely related with the STH intensity and east/ west location. It is also found that the strong precipitation in the Changjiang River valley uneasily occurs not only when STH is weak and its location is eastward of the mean for a long time, but also it connects with Iran high and anomaly westward of the mean. But the strong precipitation in the Changjiang River valley easily occurs when STH′s mean ridge line is located by the south and its west-end in 95～115°E.