The mass testing comes after the discovery of a dozen cases linked to a hospital treating coronavirus patients arriving from abroad.
In May, China tested the entire city of Wuhan – home to 11 million people and the epicentre of the global pandemic.
The country has largely brought the virus under control.
That is in stark contrast to other parts of the world, where there are still high case numbers and lockdown restrictions of varying severity.
All the cases were linked to the same hospital, said the state-run Global Times.
The Chinese authorities now have a strategy of mass testing even when a new coronavirus cluster appears to be relatively minor, correspondents say.