Local media reports that the girl's friends had 'handed her over' to two of the people dressed up as a joke, who carried her to the cart.
One of the 'witches' is said to have held the girl over the cauldron while another lifted the lid.
This was reportedly done to mimic the scene told in fairy tales and legends where witches would boil their victims alive.
It is not known how the teenager ended up with her feet in the cauldron, TheLocal.de reports.
Police said in a statement that the people involved in the carnival who dragged the cart with the cauldron simply continued the procession without caring for the victim.
She has been hospitalised and is expected to need to remain in specialist care for more than a week.
The organisation behind the carnival, Hexenzunft Eppingen - meaning 'Witchcraft Eppingen' - have been organising the annual event with hundreds of participants since 2003, according to The Local.