In the clip, the man was wearing a harness that saved his life, and managed to get back up on the line in Cleveland National Forest outside San Diego earlier this week.
The video is part of a new craze called "high lining" when daredevils try to balance themselves on a suspended length of flat webbing set up at dizzying heights. They're attached by anchors that are drilled into the rocks.
Keeping your balance isn't easy as the lines swing wildly in the wind.
There are hair-raising videos on YouTube and some national parks have forbidden high-lining over safety concerns.
John Lindquist set up the so-called slacklines at Cleveland National Forest, where highlining is allowed, and he wants everyone to know he takes safety seriously.
He claimed the line can hold 7,000 pounds and "is amazingly strong."
"We try and make sure we are as safe as possible," he told Inside Edition. "This [cable] can hold 7,000 pounds, it's amazingly strong."
Jim Galli, who shot the video of his friend falling from the slackline at Cleveland National Forest insisted to Inside Edition that what they are doing is safe and they "have a harness."