The Son of Rambow star, 17, shot the upcoming movie in Australia, and headed to a beach in Queensland during a break in his filming schedule.
But his surfing trip soon turned into a disaster.
He tells Britain's Sunday Mail, "I got hit by a surfboard and the fin cut open my head. It was terrifying because there was blood everywhere. Luckily, the cut only needed six stitches.
"But when I called Michael Apted, the director, he freaked out. He thought the cut might be on my forehead or the front of my face but luckily it was at the back and covered by hair."
And Poulter wasn't the only one who suffered during the cast's time in Australia - his co-star Georgie Henley fainted after shooting an underwater scene.
She adds, "There was a panic. They had to get oxygen for me. In hospital they checked I hadn't swallowed any water or anything."