One of the long, narrow boats packed with paddlers overturned on Saturday and another arriving at the scene also capsized, TV pictures showed.
In total, almost 60 people were tipped into the water, on a stretch of a river in the city of Guilin.
A rescue effort went on into the night with 40 survivors being pulled out.
Some 200 rescuers were sent to help, the Xinhua news agency reports.
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, according to media reports.
The practice session had been organised without notifying police, and two organisers were detained, they say.
Dragon boat racing is popular in various parts of Asia, and the Dragon Boat Festival is a traditional holiday in China, when races are held nationwide.