The unexpected increase in surface ozone concentration was found along with the reduced anthropogenic emissions during the 2019 Chinese Spring Festival in Beijing. The enhanced atmospheric oxidation capacity could promote the formation of secondary aerosols, especially sulfate, which offset the decrease in PM_2.5 mass concentration. This phenomenon was likely to exist throughout the entire Beijing–Tianjin–Hebei (BTH) region to be a contributing factor to the haze during the COVID-19 lockdown.