Containing the COVID-19 pandemic by non-pharmacological interventions is costly. Using high-frequency, city-to-city truck flow data, this paper estimates the economic cost of lockdown in China, a stringent but effective policy. By comparing the truck flow change in the cities with and without lockdown, the authors find that a one-month full-scale lockdown causally reduces the truck flows connected to the locked down city in the month by 54%, implying a decline of city’s real income with the same proportion in a gravity model of city-to-city trade. The authors also structurally estimate the cost of lockdown in the gravity model, where the effects of lockdown can spill over to other cities through trade linkages. Imposing full-scale lockdown on four largest cities for one month would reduce the national real GDP by 8.6%, of which 11% is contributed by the spillover effects.