Background Study: Asynchronous online discussions are vital venues for collaborative knowledge construction. However, the lack of appropriate instruction designs poses challenges in promoting deep and substantive engagement with the core subject matter. This paper explores how to enhance adult students' knowledge construction in the context of asynchronous online discussions at an open university in Taiwan. Objectives: This study explores instructional designs aimed at promoting online knowledge construction among adult students and overcoming barriers to this process. Methods: This study used a convergent parallel mixed methods design to collect concurrently both quantitative data from the students' online discussion postings for content analysis and qualitative data from the focus group and individual interviews, the online open-ended questionnaire, and the instructor's observation logs for thematic analysis. Then the results were merged in the interpretation stage. Results and Conclusion: The research highlights the significance of instructional designs in influencing students' level of knowledge construction during online discussions. The findings suggest that instructional designs that provide teacher-led interim summaries, pose Socratic questions, and incorporate problem-solving projects can promote higher-level knowledge construction. However, time constraints, cultural influences, and unfamiliarity with peers negatively affected higher-order knowledge construction. To address these challenges, culturally responsive and technologically inclusive instructional designs that take into account the specific challenges faced by adult learners are proposed to enhance learning outcomes through active participation. Limitation: The study's limited sample size warrants further research with a larger and more diverse cohort to validate the effectiveness of the proposed instructional designs. Another limitation is the lack of a robust theoretical base for the instructional strategies presented by the current action-research study. Further examination and broader exploration in online education are needed to prop up proposed pedagogical approaches.