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Natural ventilation is a well-known method to improve indoor air quality. Most studies did not consider the elements on building envelopes. This paper tries to investigate the horizontal elements' effect on one-sided ventilation of the windward rooms along a street canyon and study the dispersion of pollutant by using a computational fluid dynamic (CFD) model. Firstly, a three-dimensional numerical model for simulating flow and pollutant dispersion was developed by using the coded OpenFOAM and then validated against the water channel and the wind tunnel results. After the validation, four different widths of horizontal elements are applied to this present study, as well as a compared case, which is of no element (W e =0). Dimensionless pollutant concentration ̅ and ventilation rates (ACH) are analyzed. The numerical results reveal that the percentage decrease of average ventilation rate of buildings reaches 94.7% when the width of the horizontal elements (W e) is increased from 0.2m to 1.5m. Besides, the pollutant concentration is reduced by adding the horizontal elements, though, the value of We do not influence the air flow. Elements on building envelope provides a possibility to improve the quality of building environment. This study intends to learn more about the effects of the horizontal elements on ventilation and pollutant dispersion and thus may contribute to city and building designs.