This study aims to explore the application of STEM-I (STEM-Imagination) project-based learning activities and its effects on the effectiveness, processes, and characteristics of STEM integrative knowledge learning and imagination development for female high school students. A total of 72 female high school students were divided into 18 teams. Students were provided with a place to discuss, share, and integrate learning activities. A questionnaire and focus group interviews were conducted for data collection. The results show that STEM-I project-based learning activities can promote the development of diverse abilities and effectively expand STEM knowledge integration and learning for female high school students as well as enhance their imagination in the project-based activities. Additionally, the STEM-I instructional model with the process of initiation, development, expression, alternative, and links (IDEAL) is confirmed. Finally, some suggestions are proposed for future studies on STEM education and imagination integrative instruction.