<p>Task design plays a crucial role in mathematics teaching and learning and has been a central focus of recent curriculum reforms in Chinese middle schools. Transforming educators’ beliefs is essential, as these beliefs shape their ability to adopt and implement effective task design practices. This study employs a mixed-methods approach to examine how the beliefs of 15 prospective teachers (PTs) evolved after participating in a university-level course on middle school mathematics task design. Data were collected through pre- and post-course open-ended questionnaires and three task design cases, focusing on beliefs about the nature of mathematics (BNM), mathematics learning (BLM), and mathematics teaching (BTM). The findings reveal a significant shift in PTs’ beliefs, with their perspectives on mathematics transitioning from instrumentalist or Platonist views to a problem-solving orientation. Additionally, changes in BNM were found to positively influence shifts in BLM and BTM. Participants also reported that the course helped them develop a deeper understanding of the role of task design in fostering student-centred learning. These results suggest that participation in a structured course on task design serves as an effective strategy for reshaping teachers’ beliefs, thereby fostering instructional innovation in middle school mathematics education.</p>