Objective To explore the effect of pre-pregnancy body mass index (BMI) on the duration of breastfeeding, identify the high-risk population and time points of early breastfeeding cessation, and provide evidence for the implementation of targeted breastfeeding guidance. Methods A total of 370 pregnant women were selected as the research objects by convenience sampling. A research cohort was established when they were admitted to the hospital for delivery, and the breastfeeding duration within 2 years was followed up by telephone. Results The overall duration of breastfeeding was 11.0(7.0,13.0) months, and breastfeeding was interrupted rapidly within 12 months.The duration of breastfeeding in the underweight group was 2.0 months shorter than that in the normal pre-pregnancy weight group.Multivariate Cox regression analysis showed that pre-pregnancy emaciation would increase the risk of early breastfeeding interruption after adjusting for confounding factors ［HR=1.429, 95%CI(1.028,1.985), P＜0.05］.Pre-pregnancy overweight had no significant effect on the duration of breastfeeding ［HR=1.105, 95%CI(0.813,1.502), P＞0.05］. Conclusion Pre-pregnancy emaciation is a high-risk factor for early breastfeeding interruption, and the first 12 months are the time node for breastfeeding interruption.It is suggested that women of childbearing age should control their weight within the normal range before pregnancy.Health care personnel should consider the maternal pre-pregnancy nutritional status when providing breastfeeding guidance, offer personalized breastfeeding guidance, and reasonably extend the duration of breastfeeding.