Abstract: Based on the data about 10 cases of sustained fine periods and 7 cases of sustained overcast-with-rainy periods in spring time 1980—1986 in the middle and lower reaches of Changjiang River, the features are analysed of 700 hPa streamline fields and of the two flows northeast and southeast of the Qinghai–Xizang Plateau. It is found that prior to sustained fine periods, the zonal flow east of the Plateau is weaker, and the north branch flow spreads southward, thus forming the meridional streamline field and initiating the sustained fine periods. Prior to sustained overcast-with-rainy periods, the zonal flow east of the plateau is stronger, and the south branch flow spreads northward. This is the reason why the stronger interfacing motion occurs between the two flows during the sustained overcast-with-rainy periods