In this study, we investigated how teachers at a preschool in Beijing, China, implemented a government-mandated obesity intervention. We analyzed interviews with 10 teachers and two directors, the children's physical examination records, the school's menus, intervention protocol, and intervention records for 21 overweight and obese children. We found that the teachers largely followed the government mandates by modifying overweight/obese children's dietary intake and physical activity level, and initiating school-family partnerships, monitoring intervention progress, and documenting the targeted children's weight status. Noticeably, over the course of 10 months, seven children reached normal weight, two changed from overweight to obese, and 12 remained overweight/obese. Challenges of implementing the obesity intervention program include struggling with uncertainty about individual children's weight loss needs and the fragility of school-family partnerships resulting from a lack of tailoring of the intervention, inadequate training and inadequate school capacity. These challenges were the main threats to implementation fidelity. To increase the intervention effectiveness, we recommend adopting tailored intervention strategies, offering hands-on training opportunities for teachers, strengthening school-family partnership and creating more flexible policies that empower schools to make the necessary adjustments to government-mandated school-based obesity intervention protocol.