More than 210,000 people had been evacuated by late Thursday, Xinhua said, with more downpours forecast for Friday.
The torrential rains have been falling for more than a week and are the worst on the island off China’s south-eastern coast since 1961, it added.
One fisherman had been killed and three others were missing, Xinhua said.
It quoted Hainan’s armed police headquarters saying that more than 1,500 soldiers and 45 rescue boats had been dispatched for rescue work.