Air pollution in megacities represents one of the greatest environmental challenges. Our observed results show that the dramatic NO_x decrease (77%) led to significant O₃ increases (a factor of 2) during the COVID‐19 lockdown in megacity Hangzhou, China. Model simulations further demonstrate large increases of daytime OH and HO₂ radicals and nighttime NO₃ radical, which can promote the gas‐phase reaction and nocturnal multiphase chemistry. Therefore, enhanced NO₃⁻ and SO₄²⁻ formation was observed during the COVID‐19 lockdown because of the enhanced oxidizing capacity. The PM_2.5 decrease was only partially offset by enhanced aerosol formation with its reduction reaching 50%. In particular, NO₃⁻ decreased largely by 68%. PM_2.5 chemical analysis reveals that vehicular emissions mainly contributed to PM_2.5 under normal conditions in Hangzhou. Whereas, stationary sources dominated the residual PM_2.5 during the COVID‐19 lockdown. This study provides evidence that large reductions in vehicular emissions can effectively mitigate air pollution in megacities.