Saomai, one of three storms to hit East Asia in the past few days, made landfall in Zhejiang province at 5.25pm (0925 GMT), hitting Cangnan county just after officials there declared a state of emergency, Xinhua news agency said.
In one town, Longgang, nearly empty streets were awash with water, all shops were closed and the storm had already battered down trees, walls and billboards, and overturned cabs, the report said.
Storm tracker Tropical Storm Risk (www.tropicalstormrisk.com) had graded Saomai a category five "super" typhoon - its highest category - but reduced that to category 4 once it came ashore.
There were no immediate reports of casualties and Saomai would move further inland at about 20km per hour (12 mph), Xinhua said.
Chinese state media said it was the most powerful storm to hit the country since August 1956, when a typhoon hit Zhejiang, triggering a tsunami that killed more than 3,000.
"Saomai is packing winds of 216kph (134 mph) and has outpaced forecasts," Xinhua quoted Li Yuzhu, head of the Zhejiang provincial observatory, as saying.
Residents in the Wenzhou area, where Cangnan is located, had reinforced windows and doors against the storm and stockpiled drinking water and food, state television said.
Wenzhou airport had closed and hundreds of passengers were stranded because of cancelled flights, one airport manager said.
"We don't know when we will open again," he told Reuters by telephone.
Millions at risk
Wenzhou, once a prosperous port and now a manufacturing centre, has a central population of 1.3 million, but there are 7.4 million in the greater Wenzhou area.
Xinhua reported that Zhejiang had already evacuated nearly one million people, with another 569,000 people moved in the neighbouring province of Fujian, as heavy rain, strong winds and a high tide hit the area.
Typhoon Prapiroon killed about
80 people last week
Schools, theatres and stadia had been turned into shelters for the displaced, a Wenzhou official said. Factories, shops and offices had been ordered to stop all activities "unrelated to battling the typhoon".
Authorities moved 100,000 people in Cangnan alone and declared a state of emergency at 5pm as Saomai brought winds as high as 184kph over land, Xinhua said.
Much of south China has been repeatedly battered by typhoons and tropical storms this year. Hundreds have been killed by rainstorms, mudslides and floods.
Tropical storm Bilis killed more than 600 in China last month and typhoon Prapiroon killed about 80 last week.
Tropical storm Bopha fizzled to the south of Taiwan this week and another veered towards the east of Japan.
Typhoons and tropical storms are common each year in Taiwan, southeast China and the Philippines between July and October.