Abstract: The mechanism of blocking high maintenance in Mei-yu period of 1991 is examined by the E -vector.The results show that the maintenance of blocking high is associated with the divergence of westerlies momentum flux,which is caused by the dragging transient disturbance transports the westertelies momentum to the north at the north-western side of blocking high and the leading transient disturbance transports the westerlies momentum to the south at the south-western side of blocking high.