Abstract: The Yangtze Delta was studied to explore the rule of environmental pollution over urban agglomerations. Mesoscale model MM5 was used to simulate the meteorological field of January, April, July and October in 2004. The results showed that the ground level and 500 m level were mainly controlled by northwest wind in winter in the Yangtze Delta. Transport currents arriving Yangtze Delta mainly came from the inland region in the west. In spring, east wind and southeast wind were prevailing in Yangtze Delta. There was a significant transport current from southeast to northwest. In summer, south wind dominated in this area. The wind from sea to inland in Hangzhou Bay and Taihu Lake surface wind were speedy and the transport and dispersion capability was strong. In autumn, Yangtze Delta was under the influence of northeast wind. The subaerial north wind prevailed to the north of Hangzhou Bay, while the northeast wind from the sea dominated. Combined with 3-D trajectory model HYSPLIT-4, the backward trajectories in a typical pollution episode was calculated. The backward trajectories confirmed that problem of pollutant input from Yangtze Delta did exist in 500 m level.