Soulik is expected to hit northern Taiwan later in the day, before crossing the narrow Taiwan Strait and slamming into China's provinces of Fujian and Zhejiang on Saturday.
"Government departments must place saving people's lives as their top priority," Chinese state media quoted Premier Li Keqiang as saying, as officials scrambled to tackle the floods.
The Taipei city government has ordered companies and schools to send staff and pupils home early, although the financial markets will operate normally.
Taiwan's China Airlines and Hong Kong-based Cathay Pacific Airways Ltd both warned of significant cancellations and disruption of flights to and from Taiwan on their websites.
The storm will also pass close to the far southern Japanese Islands of Ishigaki and Miyako.
China has ordered fishing boats to return to port and suspended ferry links to Taiwan, official news agency Xinhua said.
The typhoon approaches as large parts of China are already being lashed by torrential rain.
At least 36 people have died in flooding in the southwestern province of Sichuan since the weekend, and 166 people are missing, the China News Service said.
State television has broadcast dramatic pictures of bridges and houses being washed away around Beichuan and Dujiangyan in Sichuan, a region that is still recovering from a massive earthquake in 2008 that killed nearly 70,000 people.
China's Ministry of Civil Affairs said flooding had also hit Xinjiang in the far west as well as Tibet and Beijing, the capital. In Inner Mongolia at least five people have died, it added.
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