Background: This study examined the use of student-teacher interactions through dynamic assessment (DA) to diagnose and promote the learner's comprehension on Chinese writing patterns. Debates over the uses of standardized tests to assess language learners' performance have been ongoing in the field of second language acquisition for many years. Concerned with the emergence of learners' new capabilities, DA aims to provide insights that are more likely to be missed by mainstream assessments. Methods: This qualitative case study explored the role of interactionist DA in the development of conceptual L2 writing skills. Three intermediate L2 Chinese learners participated in an eight-week tutoring session that focused on Chinese rhetorical styles. They then engaged in collaborative tutorial sessions with the teacher and received feedback based on the DA principles. The student-teacher interactions were recorded, transcribed, and analyzed. Results: The results revealed that teachers' dialogic mediations support learners' development in many substantial ways, such as to understand and promote learners' L2 development. All participants were able to apply appropriate Chinese rhetorical structures to complete the writing tasks. One example of the teacher's inappropriate mediation that failed to promote development was also found. Conclusion: The findings demonstrated that the interactionist DA approach promoted language development and extended learners' ability beyond a given pedagogical task. Examples of dialogic mediation also showed the ways of providing appropriate mediation that is attuned to the learner's zone of proximal development. In addition, the study has implications for writing assessment practices and future research of DA praxis.