The typhoon, packing gusts of up to 227 kilometres (140 miles) per hour, was 960 kilometres east of the island's southernmost tip as of 0300 GMT, Taiwan's Central Weather Bureau said.
Soulik is moving west-northwest towards Taiwan at about 22 kilometres per hour and could narrowly skip or make landfall in the north of the island sometime between late Friday and Saturday morning, the bureau said.
"The public must heighten their vigilance as the typhoon will certainly bring strong winds and heavy rains," a weather forecaster told AFP.
Authorities on Thursday evacuated 2,300 tourists from Green Island, off the southeastern city of Taitung, and issued a warning to ships sailing north and east off Taiwan to take special precaution.
The Hong Kong Observatory has classified Soulik as a "super typhoon" on its website, while Taiwan's weather bureau listed it as a "strong typhoon".
On the Chinese mainland, meteorological authorities maintained an orange alert -- the second-highest level -- for Soulik on Thursday, Beijing's official Xinhua news agency reported.
After hitting or passing Taiwan on Saturday Soulik is expected to head towards the coastal provinces of Zhejiang and Fujian, bringing "extremely strong" winds, it cited the National Meteorological Center as saying.
In August 2009 Typhoon Morakot killed about 600 people in Taiwan, most of them buried in huge landslides in the south, in one of the worst natural disasters to lash the island in recent years.