The typhoon is expected to strike the south-eastern coast today; it is the latest in a series of heavy storms in a particularly tempestuous season.
Saomai was expected to hit the mainland on Thursday in Fujian and Zhejiang provinces, where heavy rains were expected, the Xinhua News Agency said, citing government forecasters.
Saomai, named for the Vietnamese word for the planet Venus, would be the eighth powerful storm to hit China during this year's unusually violent typhoon season.
The storm was headed toward areas that were pounded by Tropical Storm Bilis, which killed more than 600 people last month.
Another typhoon, Prapiroon, lashed China's southern coast last week, killing at least 80 people.
More than 167,000 people have been evacuated from vulnerable coastal areas of Zhejiang, while 266,000 have been evacuated in Fujian, Xinhua said.
On Wednesday, Saomai passed across the Japanese island group of Okinawa with winds up to 144 kilometres per hour, prompting airlines to cancel 141 flights and affecting 24,000 passengers.
The storm was expected to pass north of Taiwan, where the Central Weather Bureau issued a sea warning for waters off the island's northern coast. Some Taiwanese carriers said they cancelled domestic flights.
Trailing behind Saomai toward the Chinese coast was Tropical Storm Bopha.
Bopha crossed Taiwan overnight, with sustained winds of 65 km/h, Taiwan's weather bureau said. No major damage or casualties were reported, according to the government.