A numerical simulation and a wind tunnel experiment are performed to investigate the flow field and pollutant dispersion in a two dimensional (2-D) ideal urban street canyon with roofs installed in a viaduct at different heights. Two typical roof shapes are taken into consideration: double flat (DF) and double triangular (DT). The simulation results show that when the height of the viaduct is at 0.875H–0.98H for DF and at 0.71H–0.988H for DT, the main flow field in the canyon will be reversed compared to initial flow field. This reversing phenomenon will aggravate pollution for DF and significantly mitigate pollution for DT compared to levels without a viaduct. The wind tunnel experiment is qualitatively consistent with the numerical simulation results which indicates that this reversing phenomenon is not just theoretical. A mechanism is proposed whereby the viaduct obstructing airflow is the key reason for the flow field reversion. Display omitted •Reverse flow occurs if viaduct is fixed in a 2D urban street canyon.•The flow field reversing heights with two roof shapes are acquired.•Pollution will be effectively alleviated for DT cases when flow reverses.•The flow field reversing phenomenon was verified by wind tunnel test qualitatively.