Students' problem-solving ability depends on their understanding of related scientific concepts. Therefore, the modeling and assessment of students' understanding of specific scientific concepts is important to promote students' problem-solving ability, as it can find students' understanding difficulties and explore breakthrough strategies accordingly. Inspired by the theory of knowledge integration and combined with the situational characteristics of science education in China, this study established a conceptual framework about buoyant force, which was applied to model students' different understandings of it. And based on the established framework, an assessment of buoyant force was designed and tested among 622 Chinese lower-secondary school students. Through the analysis of the test data and the interview outcomes, it was found that students' understanding of buoyant force could be divided into three levels of knowledge integration including novice, intermediate, and expert. Furthermore, the results demonstrate that an emphasis on the nature of buoyant force can be an effective strategy to help students achieve a deeper conceptual understanding of buoyant force, leading to a more integrated knowledge structure.