Recently we reported the oxygen vacancy induced structural variations of the monolayer of manganese dioxide (MnO(2)) under the electronic irradiation. In this report, we further studied the formation mechanism of oxygen vacancies in a MnO(2) monolayer under the framework of density functional theory plus Hubbard model. The effect of injected electrons on the formation of oxygen vacancies has been investigated. It is believed that oxygen is most likely leaving in the form of neutral atoms or molecules. The origination of the role of negative charges has been particularly discussed.