Euan Cusack was left with deep cuts along his neck and face after the taut line just missed vital arteries.
He was cycling near his home in Merthyr Tydfil when he rode into the hidden 6ft long rope at 15 mph.
Angry mum, Sandra, 44, said: "When I saw him I couldn't believe my eyes. He had rope burns all over his face and neck, and he'd cut his eye.
"The rope was thin and white so, of course, from a distance, you couldn't see it. He could have broken his neck or been decapitated.
"The rope could have strangled him, or he could have ended up going over the bridge and into the rapids below.
"It seems that is how stupid and dangerous some people can be. Someone had tied a rope across the bridge deliberately.
"I don't want that to happen to somebody else. I want to find out who did it to show people how stupid they really are.
"It seems like it must be pranksters. They probably think it's funny. But what's funny about this?"
The potentially lethal trap was on a footbridge over the River Taff. Police are now appealing for witnesses.
A spokesman for South Wales Police said: "This incident was reported to police and we are currently investigating.
"We would urge anyone with information to contact us."
Euan said: "I think it's really stupid what these people did, and now I've got horrible scars. I just want to find out who did this to me."