The last two decades have witnessed the gradual implementation of integrated science curriculum at the junior secondary level in China. However, in most provinces of China, the implementation is not as successful as expected. Challenges were reported, yet without fine-grained investigation, with respect to science teachers' instruction on integrated science. In this study, we aim to detect major problems by investigating the instruction of integrated science at the secondary level. Classroom observation focused on the teacher and student verbal behavior, teachers' competency of instructional organization, their presentation of instructional content, and the organization of learning activities. Findings revealed that students were provided with limited opportunities for participating and engaging in learning as science teachers were dominant in classroom talk. Teachers emphasized on the integration of knowledge within one subject (within-subject knowledge), but not the integration of knowledge between subjects (cross-subject knowledge), resulting in the unsuccessful instruction of the integrative content. What is more, teachers were inadequately competent in designing and delivering science, technology and society content, scientific inquiry and scientific experiments, which also affected the quality of instruction on integrated science.