To explore the influence of the structure of buildings with void decks on the airflow and pollutant dispersion in street canyons, CFD (computational fluid dynamics) numerical models validated by the wind-tunnel tests were employed by considering four aspect ratios (H/W) and three configurations of buildings with void decks. The results show that the increase of H/W is not conducive to the dispersion of pollutants in the street canyon without any void decks. Compared with the street canyon without any void decks, the structures of building with void decks observable improves the air quality in street canyons. The structure of buildings with void decks on both sides is more propitious to the dispersion of pollutants in the street canyon than the structure of building with a void deck on one side, and the dilution effect of the structure of upstream building with a void deck on the pollutants in the street canyon is better than that of the structure of downstream building with a void deck. The pollutant concentration in the street canyon with void decks at buildings on both sides is reduced by more than 90%. Therefore, the structures of buildings with void decks significantly enhances the airflow in the area near the ground and improves the air quality in street canyons.