Background <p>Family-based treatment (FBT) that teach behavioral techniques, focusing on integrating behavior changes into daily routines and delivered in a family-based format-targeting both child and parent-appear to be the most effective treatment for childhood obesity. This study aims to assess the efficacy of FBT for preventing overweight or obesity among children and adolescents in China.</p> Methods <p>This is a 24-months randomized controlled trial (RCT). School-aged children and their parents will be screened for overweight or obesity. 200 parent-child pairs with overweight or obesity will be enrolled and randomly assigned (1:1) into the FBT Group, and the Usual Care (UC) Group. Participants in the UC group receive the UC intervention, while those in the FBT group receive both the UC and FBT. The primary outcomes are body mass index (BMI) and normal weight (assessed as BMI) percentages during the 24-month period. The secondary outcomes are waist circumference, body composition, blood test, dietary habits, levels of physical activity, and psychological well-being during the 24-month period.</p> Discussion <p><?ColorInfoStart??>This study is the first long-term RCT<?ColorInfoEnd??> to apply FBT for the preventing overweight or obesity among children and adolescents in China. The study offers novel and immediately useful information to families, healthcare providers, and medical organizations through the development and optimization of evidence-based FBT. It also provides an opportunity to assess the effectiveness of FBT implemented in both routine clinical practice and family environments for Chinese children and adolescents suffering from overweight and obesity.</p> Trial registration <p>Chinese Clinical Trial Registry Registration number ChiCTR250 0098762 Prospectively registered on March 13th, 2025.</p>