The injured driver was sent to a nearby hospital. Police said his condition was not critical.
Investigators said the truck was overloaded, as more than 100 tonnes of steel plates were collected from the scene.
A resident who lives near the bridge said he heard two bangs and felt his home was shaking. "I thought it was an earthquake."
A surveillance camera shows that minutes before the cave-in, two trucks each loaded with over 100 tonnes of cargo crossed the bridge, said a spokesman of Hangzhou municipal government at a press conference.
The bridge and several other roads leading to the bridge were closed after the accident. At 9:45 a.m., two-way traffic resumed on the left lane of the bridge, the spokesman said.
The No. 3 Qiantang River Bridge opened in 1997 and links Hangzhou's city center with the Xiaoshan International Airport. Trucks are forbidden from crossing the bridge, according to local authorities.