A video published by state broadcaster CCTV showed the middle of the bridge's arch section suddenly giving way and plunging into the waters of the Yellow River below.
The cause was a steel cable failure, state news agency Xinhua said.
The People's Daily newspaper said 15 workers and a project manager were on-site at the time.
Twelve people have been confirmed dead and four people are still missing, CCTV reported.
The bridge on the Sichuan-Qinghai Railway is the world's largest-span double-track continuous steel truss arch bridge, according to People's Daily.
It is also China's first railway steel truss arch bridge to span the Yellow River -- the country's second-longest -- the report said.
Images published on state media show the partially built bridge with its middle section missing and two giant scaffolding towers and several cranes alongside it.
Hundreds of rescue workers were mobilised for the search and rescue operation, Xinhua said.
Industrial accidents are fairly common in China due to vague regulations and lax safety standards.
In December last year, 13 people went missing after a cave-in at a construction site for a major railway in the southern Chinese city of Shenzhen. There were no reports of survivors.
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