Meteorologists expect the typhoon to gain power after it passes over the South China Sea's warm waters and regain Category Three strength. Its expected direction will send it to Hainan Island on China's southeast coast, which has a population of nine million. Rammasun will likely make landfall by Friday.
High tides, rain and wind are expected, China's Xinhua news agency said, adding that all maritime shipping in the area has been halted.
The typhoon, whose name means "god of thunder" in Thai, was responsible for 38 deaths and the evacuation of 400,000 people in the Philippines after it passed through the Japanese island of Okinawa.
Authorities took preparations seriously after Typhoon Haiyan killed 6,300 people, largely by tsunami-like wave surges, in the Philippines in November 2013.