Beachgoers were forced to pull the man out of the water and use a makeshift tourniquet after his board's fin sliced his torso.
The Perth man in his mid-40s, identified only as Michael, is in the intensive care unit after the accident.
He had been surfing at Yallingup Beach this morning when the fin of his board cut his underarm and shoulder.
It's understood the cut was so deep it hit an artery, causing severe blood loss.
He was stabilised at Busselton Hospital before being flown to Perth for emergency surgery.
He's in a critical but stable condition.
