The National Marine Environmental Forecasting Centre said the tropical storm developed into a typhoon early today and was heading toward the northwest with the same strength or slightly gaining force.
Orange alert is the second highest level on China's four-tier colour-coded warning system for strong winds.
Son-Tinh is expected to sweep the South China Sea off Hainan province tonight.
As the Typhoon approached, waves of five to eight meters had been seen in the east of the South China Sea, state-run Xinhua reported.
The centre warned ships and people in affected areas to be careful urging authorities to take full precautions. It has forecast up to 80 cm of precipitation today along the coast of eastern and southern Hainan province and eastern coast of Leizhou peninsula.
China's four-tier warning system consists of red being the most serious, followed by orange, yellow and blue.
