The diffusive characteristics of vehicle emission pollutant and the wind field characteristics in urban street canyons depend on street canyon ration of height to width, A and the symmetry of building height.In this article, a numerical model was used to study the relationship between street canyon characteristics and diffusive and wind characteristics.It shows that when A is more than 2.1, a stable vortex formed within the urban street canyon become two vortexes.The diffusive levels of the pollutant from urban street canyons in the order of from strong to weak strength are the street canyons in which the building heights in windward are highter than that of in leeward, the building heights in windward are lower than that of in leeward, and the street canyons with a series of parallel to each others, respetively.