Across the strait, authorities in China's Fujian and Zhejiang provinces ordered back to port about 17,000 fishing and merchant ships with a total of more than 300,000 aboard, the Xinhua news agency reported on Monday.
On land, officials in the rice-growing region evacuated 600,000 with homes in the storm's projected path, Xinhua said.
Travel services were suspended and seaside hotels in Fujian closed to guests.
At 1300 GMT on Monday, Haitang was packing maximum winds of 144 kmh, down from a previous 184 kmh, and gusts of up to 180 kmh, weaker than the earlier 227 kmh, said Taiwan's Central Weather Bureau.
If the typhoon stays on its present course, it will hit China's southeastern coast on Tuesday afternoon.
"Based on our current forecast data, it should make landfall in mainland China around 2pm local time," said Daniel Wu, director of the bureau's forecasting centre, noting the margin of error was two to three hours.
The official death toll in Taiwan stood at one, a man killed by falling rocks, but the National Fire Agency said another three bodies had been found and 25 people had been injured in the storm, which was weakening as it swept southwest across Taiwan towards China's coastal rice-growing provinces of Zhejiang and Fujian.
Weather forecasters warned torrential rain would continue to hammer the island through to Wednesday, but Taipei and most local governments said business will resume on Tuesday.