Socioeconomic disparities in obesity have been observed in both childhood and adulthood. However, it remains unclear how the role of risk factors influencing these inequalities has evolved over time. Longitudinal data on 2,866 children and adolescents (6-17 years old) from the China Health and Nutrition Survey were used to track their BMI during childhood, adolescence, and adulthood. Concentration Index was utilized to measure socioeconomic inequalities in obesity, while Oaxaca decomposition was employed to determine the share of different determinants of inequality. The concentration index for obesity during childhood and adulthood were 0.107 (95% CI: 0.023, 0.211) and 0.279 (95% CI: 0.203, 0.355), respectively. Changes in baseline BMI (24.6%), parental BMI (10.4%) and socioeconomic factors (6.7%) were found to be largely responsible for the increasing inequality in obesity between childhood and adulthood. Additionally, mother's education (-7.4%) was found to contribute the most to reducing these inequalities. Inequalities in obesity during childhood and adulthood are significant and growing. Interventions targeting individuals with higher BMI, especially those who are wealthy, can significantly reduce the gap.