© 2020 The Authors This study aims to explore the relationship between ambient NO2 levels and the transmission ability (basic reproductive number, R0) of COVID-19 in 63 Chinese cities. After adjustment for temperature and relative humidity, R0 was positively associated with NO2 concentration at city level. The temporal analysis within Hubei province indicated that all the 11 Hubei cities (except Xianning City) had significant positive correlations between NO2 concentration (with 12-day time lag) and R0 (r > 0.51, p < 0.005). Since the association between ambient NO2 and R0 indicated NO2 may increase underlying risk of infection in the transmission process of COVID-19. In addition, NO2 is also an indicator of traffic-related air pollution, the association between NO2 and COVID-19′s spreadability suggest that reduced population movement may have reduced the spread of the SARS-CoV-2.