China National Radio reported at least 26 people had died in the incident in central China's Henan province but it was not clear whether that was the official death toll.
CNN quoted highway police in the province as saying at least 11 people had died.
The official Xinhua News Agency, quoting local officials, said four people died and eight more were injured in the morning explosion in the province's Mianchi County.
Xinhua said the explosion of the truck, carrying fireworks, caused a 262-foot stretch of the bridge to collapse and several vehicles on the span to plunge to the ground below. The report said at least six fallen vehicles had been salvaged from the debris.
China National Radio reported the fireworks truck was traveling east when the explosion occurred on the bridge about 30 feet from the ground. But it was not clear what caused the explosion.
Rescue and cleanup efforts were continuing. The country is getting ready for the Chinese New Year this month, which is celebrated with many fireworks.