The typhoon has already swept through Japan's southern Okinawa prefecture, injuring six people and cutting power to more than 50,000 buildings.
Typhoon Dolphin made landfall near Yuhuan in eastern Zhejiang province late Sunday AEST, China's meteorological authorities said. The storm packed maximum sustained winds of 151km/h near its centre at landfall, equivalent to a Category One hurricane on the Saffir-Simpson scale.
Authorities had moved offshore workers to safety, ordered vessels back to port and increased checks at reservoirs, mountain streams, landslide-prone areas, construction sites and tourist locations.
Torrential rain is forecast through Monday across Zhejiang, Shanghai, northern Fujian, northeastern Jiangxi, central and southern Anhui, and much of Jiangsu. Parts of central and eastern Zhejiang could receive 250-500mm of rain, according to forecasters.
After landfall, Dolphin is forecast to track westward before slowing over central and southwestern China and gradually losing strength, Wang Haiping, chief forecaster at the National Meteorological Centre, told state broadcaster CCTV.
That could prolong heavy rain and raise the risk of flooding and landslides, especially in mountainous areas and along smaller rivers.
Transport was widely disrupted across the east of the country and more than one million people were evacuated from their homes.
In addition to evacuating at least 30,300 people, Shanghai has cancelled about 1500 flights, according to flight-tracking data provider VariFlight.
A 42-year-old tourist from Guangdong, surnamed Chen, said he was stranded in Shanghai after his family's flight, scheduled to depart on Sunday, was cancelled because of Typhoon Dolphin.
He said the family had been rebooked on a flight at 10pm on Monday, although they were unsure whether it would be able to take off.
"To us Guangdong people, this typhoon is as ordinary as having a meal and feeling the wind. It's just part of everyday life," Chen said, adding the storm did not appear particularly strong by the standards of China's southern coast.
In neighbouring Zhejiang province, the city of Wenzhou relocated more than 900,000 residents and opened more than 1000 emergency shelters.
In Fujian province, authorities evacuated 98,900 people from high-risk areas after raising the typhoon emergency response to Level III, according to Xinhua. Zhejiang, Shanghai and Jiangsu have also activated Level III emergency responses.
Fujian maritime authorities said 55 coastal passenger ferry routes had been suspended, all 115 offshore construction projects halted and 290 construction vessels moved to sheltered waters.
The water resources ministry said the Qiantang, Yong, Jiao and Shuiyang rivers could see major flooding, while smaller rivers in the hardest-hit areas could rise above warning levels.
Authorities warned of a high risk of geological disasters in parts of Zhejiang, while residents in areas covered by a red mountain-torrent warning were told to follow local evacuation orders.
