John Woof, 32, added petrol from his lawn-mower to the already-lit hot coals after they started to die down.
But he suffered horrific burns to his face, shoulder, arms, chest, stomach and left leg when the fumes ignited.
His frantic fiancee and friends immediately bathed him with water and wrapped his wounds in cling film.
Incredibly, no-one else – including at least seven children – were hurt.
John's fiancée, Naomi Molyneux, 22, from Bamber Bridge, Lancashire, told the local newspaper: "I was just about to go inside the house to make my neighbour's kids a drink and I saw it blow up.
"I noticed his chest first, and then I got him inside and soaked him in towels on his face and chest and hand, before running him a cold bath and kept pouring cold water over him.
"I work in a nursing home, so I knew to do that and I rang one of my friends who is a qualified nurse, and she said to put cling film on him.
"It was horrible, it just seemed to blow up into a big flame."
Lancashire Fire and Rescue Service watch manager Steve Boyne said: "This highlights the dangers of igniting barbecues.
"People should only use the correct items for igniting.
"Clearly petrol is not the right fluid to be using. I think the barbecue was already lit but he wanted to get it going better so he lobbed petrol on it."