The lockdown policy, resulted from the fight against the spread of Covid-19, offers a unique chance to test the effect of urban human activities on air quality. The study employs the event study method to analyze data on air quality (particulate matter 2.5 (PM_2.5) and Air Quality Index (AQI)) levels and economic activity (vehicle sales growth, construction area, industrial added value, crude oil production) from 31 major cities across China. It aims to examine whether the lockdown event affects economic activities, which in turn has an effect on air quality. The findings suggest that the lockdown has a significant positive effect on air quality nationwide. The air quality has been greatly improved during the lockdown period. The placebo test verifies that when there is no lockdown, the changes in air quality over the same period are smaller. The study results will help understand the factors of economic activity that have a significant impact on air quality during the lockdown period and provide a reference for the government to formulate corresponding air pollution control measures.