Background: China has experienced rapid economic growth and reduced poverty, but the associated changes of BMI categories of the Chinese population in recent years are unclear. Methods: We collected data from two-round cross-sectional National Health Service Surveys (NHSSs) conducted in China in 2013 and 2018. All family members with BMI available from the households sampled in NHSSs were included. We analyzed the standardized prevalence and changes of obesity and overweight by year, age, sex, and urban-rural area, and further identified risk factors for obesity and overweight. Findings: 273,688 individuals were included in 2013 and 25,6304 included in 2018. The standardized prevalence of obesity and overweight in adults were 19.3% (95%CI 19.1–19.4%) in 2013 and 25.6% (95%CI 25.3–25.8%) in 2018, versus 19.4% (95%CI 19·1–19.8%) in children and adolescents in 2013 and 15.8% (95%CI 15.5–16.1%) in 2018. The standardized prevalence of obesity and overweight was 16.9% (95%CI 16.0–17.3%) in young adults aged 19–29 in 2018, 48.2% higher than that in 2013 (11.4% [95%CI 11.0–11.6%]). Although the obesity and overweight prevalence was higher in urban areas, the growth rate in rural areas was 1.8 times higher than that in urban areas. Low education levels, low-income quintiles, marital status, alcohol consumption and former-smoking status were associated with higher obesity and overweight prevalence in adults. Interpretation: The rapidly increasing prevalence in young adults may lead to the growing chronic disease burden in the future, to the detriment of recent economic gains of rural families. Funding: National Health Commission of the People's Republic of China. © 2020 The Authors