The effects of canopy size and building height on natural ventilation and pollutant diffusion in street canyons were studied using the numerical model（standard k-ε model）validated by wind tunnel experiments. Three canopy sizes and four street-canyon structures were considered, and the dimensionless concentration (K) and air exchange rate (ACH) were adopted to evaluate the pollution degree and ventilation performance of the street canyons. The results show that the change of building height and canopy size can significantly affect the air flow structure and environmental ventilation in street canyons. Here, the increase of the upstream building height and canopy size is not conducive to the environmental ventilation and the diffusion of pollutants in specific areas of the street canyons. The reduction of canopy size can enhance the ventilation performance of street canyons and improve the air quality therein. However, in some street canyons with specific configurations, the effect of canopy size change on ACH is not obvious.