It is understood the man was trapped beneath the beam from almost an hour before he was freed by firefighters.
The worker, who was then taken to hospital, is believed to be in his mid-20s.
The incident occurred at the site of a £350 million power station which is under construction at Marchwood.
Siemens Power Generation is building the gas-fired power station for Marchwood Power.
The Health and Safety Executive and Marchwood Power are investigating.