The British star - who married actor Laurence Fox in December 2007 - loves the music mogul's power, arrogance and sense of humour.
She said: "I'm in love with Simon Cowell. I just think he's so funny. There's something about someone who's that powerful, being quite smug and confident about the fact they are one of the most powerful men in the world, and not apologising for it, I love that! I love his arrogance."
The 27-year-old actress - who has a 14-month-old son, Winston James, with Laurence - even dreams about becoming Simon's wife because it would be good for her confidence.
She said in an interview with The Sun newspaper: "I think, 'What are you wearing? Why is your hair like that?' But I still think that if I wasn't married I'd like him to ask for my hand in marriage. Can you imagine? It would be the most amazing day. It would be like being knighted! You'd feel so good about yourself. "
Despite her crush, Billie insists Laurence doesn't mind having a rival for her affections - because he is equally besotted with Girls Aloud beauty Cheryl Cole, who, like Simon, is a judge on UK TV talent show 'The X Factor'.
She explained: "Laurence doesn't mind because he has the whole Cheryl Cole thing going on. So we sat there watching the 'X Factor' and fantasising about people we really want, instead of each other! He once said about Cheryl, 'anywhere, anyhow, anytime.' "I do love Cheryl too, though. She's smart and funny, and obviously, I'm just obsessed with the way she looks."