The analysis of wavelet of wind field and rainfall in the Middle and Low reaches of the Yangtze River in summer 1991 shows that these elements exhibit a significant low frequency character and periodic change:quasi-two week oscillation before mid-June 30～60d oscillation after mid-June,and the time of peak value of low frequency u wind being 2～4d prior to the that of rainfall peak. Based on the study on divergence and wind fields, it is found that after July the low frequency vortex in low-latitude moves northward while the vortex in mid-latitude moves southward.As a result,the two meet between 30～35°N,which is the major cause of two heavy rainfall in the Middle and Low reaches of the Yangtze River.