The woman, who has not been named, was badly scalded on her feet and calves after what police think was a joke that went wrong in Eppingen, Bad Württemberg, over the weekend.
The teenager is expected to remain in specialist care for more than a week as she is treated for burns.
She said to have been thrown into a bubbling cauldron by two people dressed in witches outfits, who fled the scene afterwards as the procession of 2,000 carnival witches continued.
One of the “witches” is alleged to have picked her up while the other lifted the lid so the teenager could be dropped inside.
This was apparently done to recreate in a scene told in fairytales, where witches boil their victims alive.
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German police said the two people, who were pushing their cauldron on a cart, are wanted for negligent bodily harm and failure to provide assistance.
Reports from the scene claim the teen’s friends had “handed her over” after she was picked out of the crowd by those taking part in the parade.
The weekend procession took place on cobbled streets lined with medieval timbered houses, and was attended by carnival troupes from the state of Baden Württemberg and the neighbouring French region of Alsace.
The annual carnival, organised by the Epping Witches League, started in 2003.
Organisers were criticised for posting photos on Facebook of the parade without mentioning the teenager’s ordeal.
A police spokeswoman said: “For reasons unknown so far the young woman’s legs fell into the cauldron and the 18-year-old sustained severe injuries to her legs.”
Police have said it is was most likely a joke that went wrong, but that those who dropped the woman in the cauldron may face prosecution for grievous bodily harm and failing to provide assistance.
Klaus Hotaschke, the mayor of Epping, said: “Something like that mustn’t happen.”
The procession is a highlight of the year in Eppingen.
Much of southern and western Germany closes during the carnival season, which ends in the Rose Monday parades.
The biggest of which take place in Mainz, Düsseldorf and Cologne.
The season starts on Thursday, known as Old Wives’ Day, when women roam the streets and offices to snip off men’s ties in a symbolic castration.