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In this study, uneven urban streets are explored with different standard deviations (σH = 0–0.6) through numerical simulations. The ventilation and pollutant diffusion conditions are investigated under the combined action between the thermal buoyancy force and mechanical wind force. When the thermal buoyancy is uniformly distributed, the upstream vortex gradually occupies the most area of the flow unit with the increase in σH. Three configurations of green roof systems (roof, leeward, and windward cooling schemes) are assumed to represent different thermal buoyancy distributions in the street canyon. The average air exchange rate and pollutant retention time (τ) are proposed to evaluate the ventilation efficiency and pollutant dispersion condition. The "free airflow" phenomenon occurs and gradually expands with the increase in the standard deviation of each model. The flow structure and pollutant distribution inside the urban canopy are similar in the uniformly heated and leeward cooling scheme. The windward cooling scheme presents the least local retention time in most working conditions. The roof cooling scheme aggravates the pollutant accumulation conditions inside the street canyon. This study can provide some constructive guidelines in the urban planning process and benefit the design of urban greening with different building morphologies. [ABSTRACT FROM AUTHOR]