Urban green space improves people's living environment in cities. The Chinese government has been designating National Forest Cities (NFC) for almost two decades. By taking advantage of a large panel dataset of 265 cities from 2013 to 2017 and appropriate econometric methods, we evaluate the impacts of the NFC designation. It is found that after controlling the confounding effects of other factors, the designation increases total green space by 7.5%, per-capita green areas by 10.9%, and green coverage rate by 1.0%, and it reduces PM2.5 by 3%, compared to the matched cities that have not been designated. Also, the designation's effects are heterogeneous. The designation improves the urban green space in cities with a high level of economic activity or population density, but it enhances air quality only in cities with a low level of economic activity or population density; likewise, it has a more pronounced effect on urban green space in cities where its base level is low, and in the southeastern region. These empirical results are of broad academic and policy relevance.