Television pictures showed one of the long, narrow boats packed with paddlers going over on Saturday 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, media reported.
Two people had been detained in connection with the accident, Xinhua said.
Dragon boat racing is popular in various parts of Asia and the Dragon Boat Festival is a traditional holiday in China, this year on Jun 18.