Foshan is in the industrial heartland of Guangdong province close to the financial hub of Hong Kong.
The site of the collapse was in a central area of the city beneath an eight-lane road.
An area the size of two basketball courts sunk to a depth of 6m, according to state broadcaster CCTV
The line under construction runs for 23km through the city north of the provincial capital of Guangzhou, also known as Canton.
The rapid expansion of subway networks in Chinese cities has frequently led to cave-ins and other deadly accidents, and while China has made considerable progress in improving industrial safety, scores are still killed annually in factories, coal mines and transportation networks.
Most recently, gas leaking from a pipeline at a steel mill in the southern Chinese province of Guangdong killed eight people and injured 10 on Monday.
In the deadliest recent incident, an explosion in 2015 traced to improperly stored chemicals killed at least 173 people in the port city of Tianjin, about an hour east of Beijing.