It's supposed to have star anise, cinnamon, szechuan peppercorn, clove, and fennel seed, but the dominating flavor comes from star anise and cinnamon. I personally think fennel seed can definitely be omitted because it's overpowered by star anise, and I don't think szechuan peppercorn and clove add all that much to the mix, so they can be left out, too. Ground up star anise and cinnamon, and you got yourself a very good substitute.
p.s. As far as I know, garlic, ginger, and mandarin peel are added along with the five spice powder (for "red roasted" dishes), so instead of szechuan peppercorn, clove and fennel seeds, I'd rather add garlic, ginger, and orange peel (probably easier to find than mandarin peel, I suppose).