Horrifying footage showed one of the long, narrow boats packed with paddlers capsizing around 3pm UK time yesterday.
But this morning it emerged 17 had died.
It was on a stretch of a river near the city of Guilin where the current appeared to be strong.
Another dragon boat arrived at the scene, also full of paddlers, and it too capsized, the pictures showed.
Xinhua said 57 people had gone into the water and 200 rescuers were sent to help.
The rescue effort went on late into the night.
The accident happened where two flows of the river converge causing a powerful current, the CCTV broadcaster said.
Most of those who went into the river were not wearing life jackets.
Two people had been arrested in connection with the incident.
Dragon boat racing is popular in various parts of Asia and the Dragon Boat Festival is a traditional holiday in China, this year on June 18.