Recently, integrated science, technology, engineering, and mathematics (STEM) education has gained sustained attention in K-12 settings, and engineering design-based pedagogy has become a key issue. Compared with rich research in higher education, relatively few studies are performed on engineering education in K-12 schools. In this study, we combined Conceive-Design-Implement-Operate (CDIO) model with the engineering design process (EDP), naming EDP-CDIO, aiming to promote high school students' STEM competence and compare its effects with the conventional CDIO approach. A pretest--posttest nonequivalent group design was conducted among 64 eleventh-grade students with eleven lessons. Quantitative data were collected via a pretest and posttest, and qualitative data were collected via artifacts and semistructured interviews. The repeated-measures analysis of variance and epistemic network analysis revealed that, compared with the conventional CDIO approach, the EDP-CDIO model significantly improved students' STEM knowledge, skills, and attitudes and developed more comprehensive epistemic networks in STEM competence. These findings provide a reference for K-12 STEM teachers, encouraging them to implement the EDP-CDIO model more frequently in the classroom, especially with the iterative design process.