Whenever you have a question like this where you must identify literary terms, you need to find the phrase that you are focusing on in the poem and look at the words surrounding it both before and after to try and establish the context. This will greatly help you work out what the simile is comparing and how that comparison is established. Let us look at the context of this phrase:
He sprang to his sleigh, to his team gave a whistle,
And away they all flew like the down of a thistle.
This part of the poem comes atter St. Nicholas has delivered his presents and toys and has ascended the chimney and leaps on to his sleigh and leaves. Thus the simile relates to the speed of the reindeer as they fly away and the ease with which they can fly.