The accident occurred on Phahon Yothin Road in Thanyaburi district, near a Ban Na passenger van queue opposite Future Park Rangsit shopping mall at about 4pm on Monday.
A worker at the van queue who witnessed the horrific accident said she saw the truck collide with the railings of the Rangsit fly-over. Its shipping container then slid off the truck's trailer and fell, flattening the white passenger van on the road underneath.
Rescuers had to use hydraulic rescue tools to free the body of the van driver, Nattapong Tabthongmark, 19, said Pol Lt Theeradej Sangsingkaew of Pratunam Chulalongkorn police station.
The trailer was found parked on an exit ramp, about 50 metres away from the flattened van.
Ronnachai Kidsom, 33, the truck's driver, told police he failed to negotiate a curve on the fly-over and decided to swerve right from the railing to avoid crashing off the bridge, causing his load to drop onto the road below.
Police would conduct drug tests on the man and press charges against him, Pol Lt Theeradej said.
Video by Apichit Jinakul