Background The aim of this study is to evaluate the impact of the coronavirus disease 2019(COVID-19)pandemic on breastfeeding women and to identify predictors of maternal mental health and coping.Methods Mothers aged＞18 years with a breast-fed infant＜18 months of age during the COVID-19 pandemic in Beijing,China,completed a questionnaire.Descriptive analysis of lockdown consequences was performed and predictors of these outcomes were examined using stepwise linear regression.Results Of 2233 participants,29.9％,20.0％and 34.7％felt down,lonely,and worried,respectively,during the lockdown;however,85.3％felt able to cope.Poorer maternal mental health was predicted by maternal(younger age,higher education)and infant(older age,lower gestation)characteristics,and social circumstances(husband unemployed or working from home,receiving advice from family,having enough space for the baby,living close to a park or green space).Conversely,better maternal mental health was predicted by higher income,employment requiring higher qualifications,more personal space at home,shopping or walking＞once/week and lack of impact of COVID-19 on job or income.Mothers with higher education,more bedrooms,fair division of household chores and attending an online mother and baby group＞once/week reported better coping.Conclusion The findings highlight maternal characteristics and circumstances that predict poorer mental health and reduced coping which could be used to target interventions in any future public health emergencies requiring social restrictions.