Hierarchical magnesium oxide (MgO) microspheres with high adsorption capacity of heavy metal ions and potent antibacterial activity were synthesized by an aerosol-assisted method. It is a facile, efficient and economical route for the large-scale synthesis of MgO microspheres using neither precipitants nor surfactants. The prepared MgO microspheres are made up of nanosheets with large pore volume, and they exhibit the maximum adsorption capacity of Pb(II) which is 4091 mg/g. The adsorption processes of Pb(II) are well accorded with pseudo-secondary kinetic model and Langmuir thermodynamic model. The adsorption mechanism was studied by analyzing the adsorbed products which indicate the hydroxylation of the MgO surface is crucial for chemisorption. Additionally, MgO microspheres show excellent antibacterial activity against both Escherichia coli and Staphylococcus aureus. The as-prepared MgO microspheres show promising application for the treatment of heavy metal ion and bacteria-contaminated wastewater.