The 18-year-old man accused of kicking a security guard in the head at a youth festival has pleaded guilty to assault occasioning bodily harm and trespassing, but his victim has refused to accept his apology.

Martin Fulton was silent outside Perth Magistrates Court on Monday after pleading guilty to jumping the fence at the Good Life Festival on February 27 and kicking security guard Michael Rigby in the head as he was trying to restrain another man.

The incident was caught on video and went viral shortly after the festival.

Mr Rigby's friend and fellow security guard, Shane Poulton told media outside court the 56-year-old had refused an invitation from a mediator to meet with Mr Fulton for him to say sorry, because he believed any apology would be insincere.