Conson was due to hit land late on Friday, according to the Tropical Storm Risk website.
First landfall is expected in Hainan, an island province off southeastern China where 24,000 fishing boats have been called back to port in preparation.
An orange alert for wave surges up to six metres high was issued for the South China Sea.
Typhoons and tropical storms regularly hit the Philippines, China, Taiwan and Japan in the second half of the year, gathering strength from the warm waters of the Pacific Ocean or South China Sea before normally weakening over land.
Local governments in Japan recommended on Thursday that some 300,000 people be evacuated from their homes while more than 8,000 people on the Philippines' main island of Luzon have been moved to temporary shelters.
The rains that have been lashing other parts of China showed little sign of easing, and both damage and danger continued to mount.
In eastern Jiangxi province alone, 12 reservoirs have overflowed after days of torrential rain, causing flash floods and prompting evacuation of more than 70,000 people, the official Xinhua agency said.
Some 35.5 million people across southern China have been affected by the downpours, and over 1.2 million have been relocated.