The 31-year-old from Bridgend was crushed by a mechanism on the HGV which was parked on Faraday Road, Plymouth.
Police were called to the incident at around 10:15 GMT on Thursday and the casualty was taken to Derriford Hospital, where he remains in a critical condition.
Devon and Cornwall Police have appealed for witnesses.
Officers from the CID, the Serious Collisions Investigation Unit and the Health and Safety Executive are carrying out a joint investigation.
A man has been left with potentially life-threatening injuries after being crushed while unloading a lorry.