The knowledge and skills that mathematics teacher educators (MTEs) need to have for teaching mathematics teaching is a fundamental issue that influences their preparation and development. In particular, considering that mathematics teaching comprises MTEs' teaching work, do MTEs need to have school teaching experience and, if so, in what ways does school teaching experience play a role in their teaching? In this exploratory study, we describe how school teaching experience relates to teaching prospective secondary mathematics teachers (PSMTs) how to teach mathematics, from the perspective of university-based Chinese MTEs, via in-depth interviews. The interviewees generally agreed on the importance of school teaching experience by identifying various aspects of school teaching experience that relate to the knowledge, competency, and disposition needed for teaching mathematics teaching, but they also suggested that school teaching experience requires renewal, growth, and connection with educational theories and research. This suggests that school mathematics teaching experience is necessary but insufficient on its own for teaching mathematics teaching. This study also suggests the differences in capacity between teaching-based and non-teaching-based experience in understanding of and dealing with the relation between process and product, and the relation between theory and practice involved in teaching how to teach mathematics.