The total fertility rate of the Chinese population has declined dramatically over the last three decades. Research has substantiated the causal link between particulate matter (PM) and adverse health effects. However, the impact of PM on the birth intentions or fertility behavior of the childbearing population remains understudied. The paper analyzes the impact of PM2.5 concentration (a mixture of extremely small solid particles and liquid droplets found in the air) on the second birth intentions of the Chinese floating population. We used urban migrant population matching data at the prefectural level for the analysis. The unique datasets were derived from the Chinese Floating Population Dynamic Survey in 2014 administered by the National Health Commission, the National Prefecture-level City Matching Data administered by the National Bureau of Statistics of China, and the air pollution index PM2.5 collected by the Green Peace Organization. The results show that PM2.5 concentration has a negative impact on the second birth intentions of the floating population. This impact exhibits marked regional heterogeneity: the desire for a second birth across migrant groups living in south China decreases if PM2.5 concentration goes up, while migrants coming from, and living in, north China show strong intentions to have a second birth despite an increase in PM2.5 concentration in northern cities. The results have direct implications for the Chinese government at various levels to play a vital role in making and implementing environmental policies on the mitigation of smog to effectively safeguard the health of individuals and communities and potentially raise China’s fertility rate.