Police are appealing for witnesses after the incident on Petre Street in Grimesthorpe on Thursday. The man's orange Scania Flatbed truck was parked near the junction with Petre Drive.
A green Kubota Combilift approached the offside of the truck to remove steel with a forklift.
The driver, a 32-year-old man from Thirsk, was standing on the rear trailer of the truck and fell backwards. Steel scaffolding poles are thought to have fallen on top of him.
South Yorkshire police are appealing for witnesses. The Heath and Safety Executive is investigating.