The invention discloses a preparation method and application of a MnO/C positive electrode electrocatalyst. A cheap and easily available carbon material is selected as a base material, the surface ofthe carbon material is loaded with pure-phase MnO by a simple impregnation and high temperature heat treatment process, the prepared MnO/C composite material is used as a positive electrode of a battery, and oxygen gas is catalyzed to oxidize dye wastewater at low voltage. The MnO/C positive electrode electrocatalyst not only greatly improves the mineralization ability of the dye wastewater and shortens the degradation time, but also has good cycle stability.