Objective To clarify the association of pre-pregnancy body mass index (BMI) with adverse pregnancy outcomes according to maternal age in a large and diverse population in China. Design Retrospective cohort study. Setting Guangdong Province, China. Population 669101 women participated in the National Free Preconception Health Examination Project in Guangdong Province, China, from 2013 to 2017. Methods BMI were calculated and classified into four categories according to Chinese criteria: underweight (BMI <18.5kg/m2), normal weight (18.5-23.9kg/m2), overweight (24.0-27.9kg/m2), and obesity (≥28.0kg/m2). Main outcome measures Outcomes were preterm birth (PTB), large for gestational age (LGA), small for gestational age (SGA), primary caesarean delivery, shoulder dystocia or birth injury, and stillbirth. Log-binomial models were employed to estimate the adjusted risk ratios (RRs) and 95% confidential intervals (95%CIs) for underweight, overweight and obesity. Results The incidence ratios PTB, LGA, SGA, primary caesarean delivery, shoulder dystocia or birth injury and stillbirth were 5.0%, 11.2%, 9.7%, 14.5%, 1.1% and 1.2‰, respectively. Overall, compared with normal BMI, underweight was associated with increased risk of PTB (adjusted RR 1.06, 95%CI 1.04-1.09) and SGA (1.23, 1.22-1.26); overweight was associated with the increased risk of LGA (1.17, 1.14-1.19), primary caesarean delivery (1.18, 1.16-1.20) and stillbirths (1.44, 1.03-2.06); and obesity was associated with increased risk of PTB (1.12, 1.05-1.20), LGA (1.32, 1.27-1.37), primary caesarean delivery (1.45, 1.40-1.50). These associations were different according to maternal age. Conclusion Maternal pre-pregnancy abnormal BMI were associated with the risks of adverse pregnancy outcomes among Chinese population, but the risks differed according to maternal age.