Yuzhou - which has a population of 1.1 million - saw its transport system shut down and all but essential food stores closed overnight.
It follows a similar lockdown in Xi'an, where 13 million have been confined to their homes since 23 December.
The strict measures come ahead of the Lunar New Year and the Winter Olympics due to be held in Beijing.
With exactly a month to go until the Games start, foreign ministry spokesman Wang Wenbin assured reporters China had "formulated an efficient and highly effective defence system".
As part of this system, thousands of staff and volunteers started entering a bubble on Tuesday, which will see them have no physical contact with the outside world in order to limit the spread.
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