Urbanization in China is rapidly proceeding, but rural-to-urban migration and its association with overweight and obesity is not well studied. This study aimed to explore the age at arrival, duration of migration, and the corresponding association with overweight/obesity in Yi migrants in China. A cross-sectional study was conducted in rural and urban areas in 2015 in Sichuan province, China. Demographic characteristics, lifestyle factors, and anthropometry were collected. General linear regression models were used to assess the effect of duration of migration (1-10, 11-20, 21-30, and >30 years) on body mass index (BMI). Multi-variable logistic regression was used to examine the association between duration of migration and overweight/obesity (BMI ≥ 25 kg/m2). A total of 3056 Yi people (1894 Yi farmers and 1162 Yi migrants) aged 20 to 80 years were enrolled. After adjusting for age, sex, and other potential confounders, Yi migrants had 1.71 kg/m2 (95% confidence interval CI: 1.36-2.06) higher BMI and a 2.13-fold (95% CI: 1.71-2.65) higher risk of overweight/obesity than Yi farmers. In Yi migrants, stratified by age at arrival, no significant association between duration of migration and overweight/obesity was observed in those who were 0 to 20 years old at arrival. In comparison, in migrants >20 years old at arrival, compared with the reference group (1-10 years), long-term migration (>30 years) was found to be associated with overweight/obesity after adjustment (odds ratio: 1.85, 95% CI: 1.04-3.29). Yi migrants were observed to have greater risk of overweight/obesity than Yi farmers. In Yi migrants, the risk of overweight/obesity increased according to the duration of migration, especially in those who were older upon their arrival.