The X Factor judge told Heat for the magazine's Heat Extra: TV & Pop special app that he admired Cowell's normality.
"Simon's my favourite celebrity because I know him so well and he's actually very normal," Walsh said.
"I know he has funny clothes and hair but he is very normal."
He added: "The best thing about Simon is that he is a self-made man and I like people who are self-made and get where they are on their own merit."
Walsh also said that his former talent show colleague Sharon Osbourne was his second-favourite celebrity.
"Don't cross her though," he said. "Or she'll throw water over you on live TV like she did to me!"
Watch Sharon Osbourne throw water at Louis Walsh on The X Factor live shows below:
Watch Louis Walsh talk about his top five celebrities by downloading the free heat extra app. The issue is on sale now.