Hundreds of thousands of people have been evacuated as authorities warn of rising rivers and flash floods in several provinces.
Typhoon Gaemi has reached southeastern China after passing through the Taiwan Strait.
The typhoon has prompted warnings of river flooding, flash flooding and waterlogging in cities and provinces that were hit by extreme rainfall several weeks ago.
The third and most powerful typhoon to hit China's eastern coast this year made landfall in Fujian province at 7:50 p.m. (1150 GMT) on Thursday after lashing Taiwan with gusts of up to 227 kilometers per hour (141 miles per hour), some of the strongest winds recorded in the western Pacific Ocean.
Before its arrival, 240,800 people in Fujian were evacuated.
Despite weakening slightly since making landfall in Putian, a city in Fujian with more than 3 million inhabitants, Gaemi and its giant bands of storms are forecast to unleash heavy rains in at least 10 Chinese provinces in the coming days.
Gaemi's arrival has been compared to last year's Typhoon Doksuri, which caused historic flooding as far north as Beijing and caused nearly $30 billion in losses nationwide.
Authorities said water levels in the lower reaches of the Yangtze River as well as the vast freshwater lakes of Poyang and Dongting in central China could rise, returning to dangerous levels seen in early July after heavy summer rains.
Beijing has warned that due to its high vapor content, Gaemi could bring heavy rain to the Chinese capital, about 2,000 kilometers (1,242 miles) north of Putian, even if the storm weakens to a tropical depression.
Authorities warned that Gaemi's rains could cause flash flooding and waterlogging, particularly in parts of northern China, where the ground remains saturated after being hit by a storm system earlier this week.
Gaemi had intensified seasonal rains earlier this week in the Philippines, where the death toll rose to 22 and at least three people were missing, police said.
The Philippine Coast Guard said an oil tanker, MT Terra Nova, carrying about 1.4 million liters (370,000 gallons) of industrial fuel, sank off Bataan province early Thursday and rescuers saved 15 of the 16 crew members.
Gaemi causes deadly destruction in Taiwan
In Taiwan, Gaemi killed three people, caused flooding and sank a cargo ship after making landfall on Wednesday night.
Parts of southern Taiwan are expected to have seen 2,200 mm (87 inches) of rain since Tuesday. The storm knocked out power to about half a million households, although most have now been restored, utility company Taipower said.
In addition to the three fatalities, the typhoon in Taiwan injured 380 people, the government said.
Taiwan's fire department said a Tanzanian-flagged cargo ship with nine Myanmar nationals on board sank off the coast of the southern port city of Kaohsiung.
Three of them have since been found alive on the shore, Taiwan's coast guard said.
Taiwanese television stations showed images of flooded streets in cities and counties across the island.
Offices, schools and financial markets were closed for a second day on Thursday. Trains were halted until 3 p.m. (0700 GMT) and all domestic and 195 international flights were cancelled.
