The crash involved a single vehicle which had left the road and crashed into a bush.
Police were called at 12.52pm and closed the eastbound carriageway between junction 15 at the B663 Toll Bar Lane at Keyston and junction 16, Fox Road at Catworth.
Firefighters from Huntingdon and Rushden used specialist stabilising equipment and hydraulic cutting gear to rescue a man who suffered serious injuries.
Traffic was diverted onto the A605 at Thrapston, towards Peterborough.
The road has now reopened.