The early working title was Strawberry Milk, a drink which Bo-ah Jo's character has a fixation on. It was changed to Thorn because Tae-gyun Kim said, "There is nothing sweet and soft about the movie like strawberry flavored milk. Only sharpness like a thorn exists. It represents something difficult."

Filming began on September 14, 2013, at Jijok Middle School in Daejeon. Shooting wrapped on November 10, 2013.

Auditioned 250 actresses, before casting Bo-ah Jo in her big screen debut.

The film reunited director Tae-gyun Kim and actor Hyuk Jang for the first time in thirteen years since Hwasango (2001).