China faces severe air pollution issues after the rapid growth of economy, causing human physical and mental health concerns as well as behavioral changes. Such adverse impacts can be mediated by individual avoidance behaviors such as traveling from polluted cities to the cleaner ones. This study utilizes smartphone-based location data and instrumental variable (IV) regression to fill the gap of how air quality affects population mobility. Our results confirmed that air quality does affect the population outflows of cities. The increase of air quality index (AQI) by 100 points will cause a 49.60% increase in population outflows, and a rise of 1 µg/m3 of PM2.5 may cause a 0.47% rise in population outflows. Air pollution of former days can drive people to leave their cities three days or a week later by railway or by road. The effect is heterogeneous among workdays, weekends, and holidays. Our results implied that air quality management can be critical for urban tourism and environmental competitiveness.