At the end of December 2019, Coronavirus Disease 2019 (COVID-19) quickly spread throughout Hubei Province and other parts of China. During the 2020 Spring Festival, public activities were canceled, people tried their best to stay at home, and human and industrial activities were reduced to a basic or minimum level. However, during this period, severe fog-haze events occurred over the North China Plain. What was the leading factor that caused these severe smog incidents? And what were the individual impacts of meteorological conditions and emission reductions?