A series of studies have proved that green space can influence air pollution, which is the main risk factor for asthma. In addition, the biodiversity hypothesis suggests using green space contributes to increasing human commensal microbiota on the skin, which is possible to enhance the immunity to allergic atopy. Given that, the co-effects of green space might jointly influence asthma. However, existing studies mainly focus on one or part of the pathway between them, and most of them shed light on the influence of greenness or trees on asthma. Green space structure that can play a role in influencing both air pollution and microbial diversity is also probably a sig-nificant factor to influence asthma. Based on that, the present study takes Toronto as the case to explore two potential mechanisms between green space structure and asthma. Besides, tree diver-sity that might moderate the influence of green space structure on asthma was also considered in this study. After regression analysis was applied in SPSS, the impact of green space structure on the prevalence of asthma by reducing air pollution was not found in this study. Moreover, green space structure has no significant relationship with the prevalence of asthma, but when moder-ated by tree diversity, it has the potential to reduce the prevalence of male asthmatics and such moderation effect of tree diversity on green space structure only works in male asthmatics. Except for that, several covariates selected in this study were also found to correlate with the prevalence of asthma. In particular, the relationship between household income and the prevalence of asth-ma only significantly exists in female asthmatics.