You have not given any information to go along with this, so I will do the best I can.
It is not normal for a bird to bite it's feet. If you have had this bird for some time, and this is a new behavior, you probably realize it is not normal and this is why you wrote us.
Please Find an Avian Vet to examine your bird to determine what is wrong with your bird. It could be it has an internal infection that is causing this behavior.