The purpose of this research is to study the development of Mathematics Teacher Educators (MTEs)’ Pedagogical Content Knowledge (PCK) through the implementation of Thailand Lesson Stuidy incorporated Open Approach (TLSOA) model (Inprasitha, 2022) at the Savannakhet Teacher Training College Demonstration Primary School during the 2023–2024 academic year. This qualitative research utilized a Case Study design involving a sample of three MTEs, they were selected through a purposive sampling method. The data collection instruments included field notes, lesson plans, and video recordings. The data were analyzed through content analysis. The findings indicate that the cycles of TLSOA significantly developed most components of the MTEs' Pedagogical Content Knowledge. In Step 1, the MTEs enhanced their instructional planning and subject matter knowledge by improving their ability to design problem situations that closely connected with students' real-world experiences and prior knowledge. This led to a greater capacity to anticipate student ideas and potential difficulties in problem-solving. In Step 2, the MTEs developed their teaching methods, learning to represent content effectively by presenting engaging problem situations that captured students' attention, prioritizing student ideas, and asking meaningful questions to facilitate better classroom discussions. Finally, in Step 3, MTEs further deepened their Content Knowledge and Curriculum Knowledge by developing the ability to clearly explain students' ideas in various problem situations and recognize specific misconceptions about the mathematical content. This enhanced understanding was then effectively reflected back into the design of lesson plans and instruction that emphasized students' thinking and problem-solving processes.