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STEM-based engineering curricula is promoted in K-12 education. Engineering design is a critical element of engineering curriculum and a competency that students in STEM fields need to acquire. We used a robot design course to investigate high-school students’ understanding of engineering design activities. Reflective practice was conducted after the robot design course. We collected 181 dialogues of students’ reflections and the dialogues were coded as ten engineering design activities. There were few dialogues could be coded as engineering design activities. Eight self-iterative behaviors were significant and two sequential behavior transfers of were significant. The outcomes show that higher-order engineering design thinking skills take a longer time to develop than technical skills. For high-school students, it is important to enhance their higher-order engineering design thinking skills to promote higher engineering design performance. We suggest provided high-school students with convenient processing tools and easy-to-use, hands-on techniques to increase their technical skills. Students therefore have more time to spent their efforts on building design thinking skills. Finally, we suggest that educators from institutions of higher education and K-12 schools should work together to develop pedagogical models that provide rigorous, well-rounded education and outstanding engineering design instructions to most effectively cultivate STEM talent.