A 50-year-old man from Wolverhampton was arrested at a HS2 site in Fradley Wood, near Lichfield, on suspicion of assault and possessing an offensive weapon.
He has since received a conditional caution. Staffordshire Police says it is probing the treatment of the man by HS2 security staff, following complaints from members of the public.
The man is understood to have been restrained by security staff before police arrived on the scene.
Lichfield MP Michael Fabricant has raised the issue with the force’s Chief Constable Gareth Morgan, saying he was concerned that HS2 was using “heavy handed” tactics when dealing with protesters.
A spokesman for Staffordshire Police said officers were called to an incident at about 11.30am on November 15, following reports a man had fired pellets at staff.
"Officers attended the location and a 50-year-old man from Wolverhampton was arrested on suspicion of assault and possessing an offensive weapon," said the spokesman.
"He has since received a conditional caution. Following the incident we have been made aware of complaints about how the man was restrained by security staff and we are investigating.”
Mr Fabricant said he had been contacted by an observer who claimed the protester had used a catapult to fire carrots over the HS2 fence. Images of other protesters being restrained have also been sent to the MP.
A spokesperson for HS2 said: “In recent weeks, HS2 has been repeatedly and excessively targeted by political activists who have trespassed on construction sites, endangering themselves, our workers and the general public, subjecting our hard-working employees to violence and anti-social behaviour.”