Category: Other; Midfoot/Forefoot Introduction/Purpose: There is a greater prevalence of flexible flatfoot (FFF) among children; however, most of them are physiological, and the associated factors are still in debate. The purpose of current cross-sectional study was to investigate the FFF prevalence of 6 to 13 years Han nationality children in Northwest China; and to investigate the potential relative factors correlated with FFF. Methods: A total of 1,059 children aged from 6 to 13 years were included. Dynamic footprints according to FootScan system were collected from both feet. The relationship of FFF with age, gender, side, body mass index (BMI), foot length-width ratio (LWR), and contact area were investigated. Results: The percentage of FFF decreased from 39.5% at 6 years to 11.8% at 12 years, and reached a platform at 12 to 13 years. Overweight (OR 1.35, 95%CI 1.03-1.77) and obese (OR 2.43, 95%CI 1.81-3.26) showed positive correlation with percentage of FFF children; but only obese (OR 3.25, 95%CI 2.19-4.82) showed positive correlation in small age group (6 to 9 years). The mean LWR was larger in healthy children, and still larger in small and large age groups (P < 0.01). The mean midfoot contact area percentage was larger in FFF than healthy foot (P < 0.01). No correlation was found between FFF prevalence and gender or side. Conclusion: The prevalence of FFF decreases with age, and reached a platform at 12 to 13 years. The prevalence of FFF is positively correlated with increased BMI, especially in small aged children; and the prevalence of FFF is positively correlated with decreased LWR.