This study utilized action research to conduct planning, implementation, and reflection on a teaching internship. The objectives of this study were to understand the implementation of integrating a lesson study with a teaching internship course as well as pre-service teachers’ learning effectiveness. The participants were 36 elementary school pre-service teachers. Interviews, observations, and documents collection were conducted for data collection. After data analysis, the following conclusions were obtained. First, the lesson study was conducive to the practice of the teaching internship course through the processes of an introduction to ideas, microteaching and intensive internship. Second, pre-service teachers gradually mastered the design and implementation of teaching during microteaching, bur there were still difficulties in understanding elementary school students. During the intensive internship, it was possible to deepen the effectiveness of pre-service teachers’ teaching design and implementation and to better understand elementary school students’ learning. The suggestions of this research are to guide pre-service teachers to revise their teaching designs after the intensive internship. This study also advises to further explore the situation of pre-service teachers’ participation in the lesson study in regards to teaching materials, methods course, or educational practicum.