China's National Meteorological Centre (NMC) said Super Typhoon Rammasun was on course to hit Hainan island and Guangdong province late in the afternoon.
The outer bands of the storm lashed Hong Kong overnight with heavy rain and strong winds but the city was spared a direct hit as the typhoon veered west towards Hainan.
Packing winds Friday morning of up to 198 kilometres an hour, the super typhoon was expected to bring torrential rains, the NMC said.
China's official news agency Xinhua said Rammasun was expected to be the strongest typhoon to hit Hainan in 40 years.
On Thursday, the NMC issued its highest "red alert" for the storm, its first such declaration this year according to Xinhua.
The typhoon comes after dozens of people died in the past week as heavy rain battered swathes of China, with at least six killed by lightning, thousands of homes destroyed and more than 300,000 evacuated, state media have reported.
Earlier, Rammasun -- a Thai word for "Thunder God" -- hit the Philippines, slamming the capital Manila, killing at least 40 people, and leaving millions without power.
The Philippines is often the first major landmass to be struck after storms build above the warm waters of the Pacific Ocean.
The typhoon destroyed or damaged 26,000 homes, while cutting electricity supplies to nearly all of Manila, a megacity of more than 12 million people, and surrounding urban areas.
Rammasun was the first typhoon to make landfall in the Philippines since this year's rainy season began in June, and the first major storm since Super Typhoon Haiyan devastated the eastern islands of Samar and Leyte in November last year.
Haiyan killed up to 7,300 people in one of the Philippines' worst natural disasters, but this week's typhoon followed a different track.