Chinese media say there have been 18 cases in the county since the beginning of the recent surge in Beijing two weeks ago.
The lockdown reportedly impacts around 4 lakh people.
Officials announced that in Anxin country, which is located 150 kilometers from Beijing, only essential workers will be allowed to leave their homes.
No non-residents will be allowed to enter buildings, communities or villages, according to the BBC.
Almost a third of the cases so far have been linked to one beef and mutton section in the market, where workers are being made to quarantine for a month, city officials said Sunday.
Businesses in Anxin county had supplied freshwater fish to the Xinfadi market, state news agency Xinhua reported.
While this is a small number compared with the thousands of daily cases in the US or South America, China reacted quickly to contain any spread.
China reported 12 new confirmed COVID-19 cases as of the end of June 28, down from 17 reported a day earlier as the country's capital tries to curb a new wave of infections that emerged from a wholesale market in mid-June.
The rest of China has seen only a small trickle of single-digit infections, most of which were imported cases, which means travellers returning from overseas.
The NHC said in a statement that five of the new COVID-19 cases were imported infections involving travellers from overseas, compared with three such cases reported a day earlier.
The COVID-19 pandemic originated in China at the end of past year but the number of new infections has since dropped dramatically.
In general, China has successfully "flattened the curve" in recent months.
It's been reported that the new cases in Beijing go against predictions by the National Health Commission expert Wu Hao.