Objective: To assess plantar arch index (AI) and prevalence of flat feet in school-age children (7-12 years old) in Shanghai and evaluate the relationship between flat feet with age, gender, weight status (BMI) and occurring sides, and to provide a reference for the prevention of flatfeet. Methods: Three-dimensional foot measuring instruments were used to measure bilateral foot length, medial arch height, AI and arch height ratio (AHR) in school-age children in Shanghai. Statistical analysis of these foot parameters was performed. Results: A total of 3 226 children between aged 7 and 12 in Shanghai were measured with AI of (0.27±0.05) and AHR of (3.02±1.89). Prevalence of flat feet in the group of 7 to 12 year old children was 56.1%. Prevalence of flatfeet decreased significantly with age: 72.6% at 7 years old and 37.9% at 12 years old. Boys had a significantly greater risk for flat feet than girls: the prevalence of flat feet was 62.9% for boys and 47.8% for girls (OR=1.81, 95%CI=1.57-2.10). This risk was independent of age but related to gender. The risk of flat feet in boys was always higher than that in girls at every age. For children aged 7-8, being overweight was not significantly related to the occurrence of flat feet. However, for children aged 9-12 who were overweight were more likely to have flat feet than those of normal weight. The OR increased with age: from 1.44 (95%CI=1.03-2.03) at 9 to 2.96 (95%CI=1.68-5.23) at 12. There was no difference on which side flat feet would occur (χ2=0.95，P=0.33). Conclusion This finding shows that prevalence of flat feet is influenced by age, gender and weight status. AI and prevalence of flat feet in children aged 7-12 decreases with age, and boys have significantly higher prevalence of flat feet than girls. Overweight children aged 9 or older have a higher risk of flat feet.