The first train, traveling south from Zhejiang's provincial capital of Hangzhou, had stalled after being struck by lightning. That's when the second train hit it from behind.
The impact dislodged four cars, causing them to plunge about 65 to 100 feet from the elevated section of the track. One carriage ended up in a vertical position, leaning against the viaduct.
The trains involved in the crash travel at an average speed of 95 mph.
It was the first derailment on China's high-speed rail network since the country launched bullet trains with a top speed of 155 miles (250 kilometers) per hour in 2007, the China Daily reported.
The Associated Press contributed to this report.