Twenty five vehicles were buried in the collapse, said the official Xinhua News Agency. It said nine people were confirmed dead and another 13 injured, including four in serious condition.
Reports by China National Radio of 26 people killed were removed from websites.
An 87-yard stretch of the east-west highway collapsed in Mianchi county in Henan province. It scattered blackened chunks of debris and shattered windows in a nearby truck stop.
A truck driver interviewed on CCTV said he was only 20 yards away from the explosion.
"I heard a huge bang and immediately braked. I saw small fireballs falling down one by one," said the lorry driver, whose truck windscreen was smashed.
"I then heard the sounds of clanking and exploding for five to six minutes. My face was covered in dust."
There was no immediate word on the cause of the explosion. It occurred west of Luoyang, an ancient capital of China known for grottoes of Buddhist statues carved from limestone cliffs.
Fireworks are a popular part of Chinese Lunar New Year festivities. To meet the demand, fireworks are made, shipped and stored in large quantities.
In 2006, a storeroom of fireworks exploded at a fair in Henan, killing 36 people and injuring dozens more.