Abstract: A diagnostic study of the average characteristics and maintenance mechanism of the blocking situation in East Asia during the drought period in summer of 1980 (1 July－10 September) has been made by use of quasi-geostrophic potential vorticity equation. It has been shown that the blocking high in East Asia is a kind of deep and warm anticyclonic system of unusually low potential vorticity. On the one hand, its maintenance is related to the eddy transport resulting from the long wave trough in the Ural region upstream, on the other hand, and more importantly, it is associated with the transport of the air of low potential vorticity originating in the Tibetan Plateau. The estimate of budget of potential vorticity has further indicated that the eddy forcing term is nearly balanced out by the advection term of the mean flow. This face implies that the eddy transport of potential vorticity may be an important factor responsible for maintenance of unusually low potential vorticity in the blocking area and prohibiting the blocking high from the downstream advection.