Chinese state media said Conson made landfall on the southern tip of the island province of Hainan.
The storm is expected to add to the worst floods in China in more than a decade, which have already killed 135 people so far this month.
More than 35 million people across China have been hit by the poor weather and 1.2 million have been relocated.
Conson is expected to cross more of south-western China before arriving in northern Vietnam, where about 200,000 people have fled their homes.
