Jamie Poulton, 45, reportedly suffered second-degree burns as he plucked eight-year-old Matilda from a group of children and put out the flames with his bare hands.
The restaurant owner from London said he was just a few feet away when he saw the youngster's black wizard outfit turn into "a crackling ball of flame" on a narrow doorstep.
He told the Daily Mail: "All the children were wearing the same kit. It was like a potential horror film in front of me, because they were all going to go.
"This material just keeps reigniting and re-burning. And it is sticky, so it melts on the skin. It doesn't cinder. It remains hot. It was horrific to be honest."
Mr Poulton, a father of two, insisted he was not a hero.
"Any parent would have done exactly the same thing so I don't think there is anything heroic about it," he told the newspaper.