A fine-mesh regional meteorological model that suits the Yangtze Delta region was developed. This model was used to simulate some micro weathers, such as the land and sea breezes, land and lake breezes and urban heat island effect, in the Yangtze Delta successfully. The model was also used to study the characteristics and the formation mechanism of surface shear line in the region. It is indicated that the interaction between the sea breeze and the lake breeze is the main factor for the formation and maintenance of the surface shear line.