Contact was lost with the flight over Wuzhou,
in the Guangxi region, authorities said.
It was scheduled to fly from Kunming to Guangzhou in the southeast of the country.
There were 132 people on board
including 123 passengers and nine crew members.
The Boeing 737-800 was cruising at 29,100 feet
and began a sharp descent. It recovered briefly and then resumed its dive before it lost contact.
It fell more than 25,000 feet in about two minutes.