The Spring Festival holiday is over but chunyun, or the spring rush, will continue to February 16, as tens of millions of people head back to the cities where they work or study after spending time with their families.
However, rainfall and snow in some parts of the country is expected to affect their journeys.
In most parts of southern China, rain and snow is forecast during the upcoming peak.
Shanghai Railway Bureau, the rail operator of eastern China, estimates that the first wave of passengers returning to the city will last until Thursday.
More services have been added to deal with the numbers of passengers expected.
More than 6.65 million passengers traveled by train on Saturday, the last day of the holiday but many had decided to return earlier.
"I brought with me three fellows from hometown," said a man, surnamed Li, a Jiangsu Province native who works in Shanghai as a mover. Local moving companies are short of workers, Li said.
He arrived in Shanghai last Thursday by bus, after a short reunion with his family.
Shanghai's long-distance bus stations said many migrant workers make the city as a transit point to industrial zones close to Shanghai in the Yangtze Delta region such in as Jiangsu Province's Kunshan and Taicang.