Integrated STEM education has been widely promoted to cultivate students' competencies to solve real-world problems. To promote STEM education, it is crucial to design high-quality STEM curricula. However, STEM curriculum design is a complex task involving multiple components and complicated processes to coordinate the components into a coherent design. Existing STEM teacher training programs lack sufficient guidance on how to deal with the complexity in STEM curriculum design, posing challenges to STEM teachers. To address the gap, this study proposed a conceptual framework that visualizes the complex process of STEM curriculum design. The conceptual framework includes (a) a process model for STEM curriculum design, with key strategies and guidelines for each step, and (b) a scaffold for designing STEM teaching and learning activities. An example is presented for illustration. Using the proposed approach, a training program on STEM curriculum design was provided to 27 pre-service teachers. After the training, the participants made a significant improvement in their STEM curriculum design performance, especially in the design of teaching and learning activities. Participants' comments highlight the benefits of the proposed approach, in particular its provision of a systematic framework and clear guidelines for STEM curriculum design. The study demonstrates the feasibility and promising potential of the visible thinking approach in supporting teacher learning of integrated STEM curriculum design.