Abstract: Objectives: Before the comprehensive transition to the era of low-carbon travel, the air pollution in urban canopy layer (UCL) directly caused by vehicle exhaust emission has always been a hot topic to deal with urban atmospheric environment. Methods: This paper constructs a set of three-dimensional physical models of typical urban streets under different street interface density (SID), and uses the computational fluid dynamics (CFD) numerical simulation methods to study the influence of SID on air flow and pollutant diffusion. The indices including air exchange rate (AER), purification flow rate (PFR), average residence time (ART) and pollutant concentration at the pedestrian area were used to evaluate the natural ventilation performance and pollution diffusion capacity of urban streets under different SIDs. Results: The results show that the CFD simulation methods can better represent the paradigm of air flow and pollutant diffusion in urban street space. The reduction of SID on either side of the urban street is conducive to the optimization of its natural ventilation and air quality. Compared with the downstream of urban street, the reduction of SID on the upstream has a more significant effect on the improvement of air pollution. The layout of buildings on both sides of the urban street has a ‘Critical Effect’ on the overall ventilation performance and pollutant diffusion capacity of urban streets under different SIDs, and the critical upstream SID is about 0.8. Conclusions: CFD simulation can present the detailed process of air flow and pollutant diffusion, which can promote the scientific planning and design control of urban street space, and provide active response strategies for residents near urban streets to deal with traffic related air pollution.