Effective teaching of STEM has become a significant concern for teachers' professional development (PD). Many teachers are not familiar with STEM strategies and do not possess the disciplinary knowledge or pedagogical strategies demanded by STEM teaching. The purpose of the study reported in this paper was to explore the path and model of Chinese teachers' professional development about STEM understanding based on their participation in the Zhejiang-Indiana STEM summer programme. This programme capitalises on strategies referred to as the "observation-discussion-reflection" (ODR) framework. The participants were 82 teachers who specialise in science, technology, or mathematics and who attended the training activities. There were two weeks of training interventions. Statistical analysis of two groups of teachers' pre-test and post-test, as well as the comparative analysis of two groups, were conducted. The changes in teachers' STEM discipline knowledge as well as their abilities and attitudes were investigated. Overall, the findings indicate variations in how teachers understand the different components of STEM knowledge with integrated knowledge requiring the most attention for further professional development.