Tom Ciotkowski, a council worker from Stratford upon Avon, could have faced up to five years in jail over the incident which took place in July 2018.
His lawyers hailed Thursday's verdict from the court in Boulogne-sur-Mer as “thwarting an attempt to criminalise a caregiver”.
According to France's CRS riot police, they were trying to remove migrants from an area near the ring road around the northern city of Calais when a group of “outspoken” British volunteers turned up.
They said Ciotkowski had “shoved (the policeman) in the chest” and called him a “bitch bastard”, prompting the officer to push back in self-defence, causing him to fall over a security barrier.
But their account was challenged by the defence and by Amnesty International, which accused them of putting him on trial “on trumped up charges”.
According to Ciotkowski, he was passing in a vehicle when he noticed police arguing with a group of volunteers so he got out to film what was going on.
In footage of the incident released by Amnesty, Ciotkowski explains how a policeman twice kicked a volunteer then hit a woman next to him. As he asks for the officer's badge number, there is a scuffle and a policeman can be seen shoving a figure over the barrier, almost into the path of an oncoming lorry.
“I felt lucky that I hadn't died really. None of us had done anything wrong,” said the 30-year-old, who has pledged to return to the northern French city to keep on helping refugees gathered there in the hope of somehow reaching Britain.
Amnesty hailed his acquittal, which came on World Refugee Day, as “a victory for justice but also for common sense”, saying the actions of the police “should be promptly and thoroughly investigated”.
Earlier this month, Amnesty accused the French authorities of harassing, intimidating and even assaulting those offering aid to migrants in northern France in a deliberate bid to discourage their work.
Last month, Ciotkowski filed a complaint against the police officer who pushed him and against others who backed him up, with the IGPN police disciplinary body due to present its findings by the end of the year.