Traffic pollution emission has drawn people's attention to the air quality of pedestrian height in street canyon, and many studies reported the negative effect of vegetation on air quality. In previous studies, numerical simulation was used to control variables to explore the relationship between pollutant concentration and individual characteristics or layout parameters of plants in urban street canyon, quantify the influence of plants in the meantime. However, the relationship between pollutant concentration and complex greening conditions in wide street canyon with real street geometry has not received enough attention up to now, and few studies have evaluated the combined ecological effects of such kind of street green spaces on air quality and microclimate. Therefore, this paper selected some street structure factors and greening factors on green space structure and total quantity, discussed the effect of street green space on air quality and thermal environment of slow lane, from how the factors were correlated to these dependent variables to how much effect it had, especially focusing on air quality of slow lane, and evaluated the comprehensive ecological benefits brought by greening. In Beijing, nine sample areas with different greening structure under certain differences in urban geometry were selected, in conjunction with no-greening and current greening generated a total of 18 scenarios for ENVI-met modeling and analysis. The results showed that under the complex street structure and greening structure, the change of slow lane air quality caused by greening could best be characterized by the greening factor called near-source crown volume fraction (NCVF). Street greening had both positive and negative ecological effects, as shown in the increase of PM_2.5 concentration in slow lane by 9.34%-99.70% and the decrease of temperature by 1.06℃ on average. The near-source crown volume fraction was significantly positively correlated with the change rate of concentration, while the green coverage rate and sky view factor (SVF) of slow lane were significantly correlated with the change rate of temperature, humidity and wind speed. The scenes with medium green coverage rate in each road grade have the best comprehensive ecological effect, which in the main and secondary roads are characterized by arbor-shrub structure on the greenbelt between motor way and bicycle lane, and uniform type of the horizontal structure. The research promotes deep understanding of the ecological function trade-off of multilayer greening in broad street canyons, and provides basis for street greening allocation method and ecological benefit evaluation.