The purpose of this research is to describe the development of a STEM-based learning environment for early childhood teacher candidates that is valid, practical, and has potential effects. The research sample consisted of 25 pre-service early childhood teachers and 19 students Kindergarten. The research method used was design research with the type of development study. This research has produced a learning environment with the Campus-Application-School (CAS) model consisting of the stages of training, designing, mentoring, and testing. The validity of the STEM-based learning environment model was seen from the developed student worksheet (LKPD) instrument and the content in the learning environment, obtaining a value of 0.88, which was categorized as high validity. The practicality, as seen from the results of one-to-one teaching, small group, and field tests with an average of 94.2, was categorized as very good. Meanwhile, the potential effect was based on Kirkpatrick's four levels of training evaluation with an average of 83.73, which was categorized as very good. This research suggests that the CAS learning environment model can affect teacher approaches to teaching in a step appropriate way based on STEM to prepare student teachers to teach basic mathematics.