Reports said 151 people were under observation in the city of Yumen in Gansu province after authorities determined they had come in contact with a man who had died of the plague on July 16.
Investigators believed the man had contracted the bacterial infection after contact with a marmot.
The report said all the people under quarantine were in good health but 10 checkpoints were still blocking off parts of the city.
Bubonic plague killed millions of people in Europe in the 14th century and tens of thousands in China in the 19th century.
It is spread through flea bites and can cause gangrene, seizures and fever.
Meanwhile, police in Shanghai have detained five people in an investigation into a Chinese-based supplier of foreign fast-food brands including KFC and McDonald's over allegations the firm supplied out-of-date meat. Food which had fallen on the floor was also said to have been used by the firm.