The invention discloses a manganese dioxide-magnetic biochar composite catalyst as well as a preparation method and application thereof. The preparation method comprises the following steps: 1) pyrolyzing biomass under anaerobic or oxygen-limited conditions to generate biochar, washing with water and alcohol for three times respectively, drying, grinding and sieving; 2) uniformly stirring a ferricsalt and a ferrous salt in an aqueous solution, then adding the prepared biochar, and then adding an alkali liquor; under heating, conveying an obtained coprecipitate into ferroferric oxide, and obtaining magnetic biochar; washing with watert for three times, drying, grinding and sieving; 3) adding the magnetic biochar into a manganese salt solution, and uniformly stirring; then adding a potassium permanganate solution, uniformly mixing and heating at a high temperature for a period of time. The method is simple in process, low in cost, and is capable of achieving large-scale industrial production. The prepared manganese dioxide-magnetic biochar composite catalyst is excellent in catalytic performance, can be applied to degradation and removal of various pollutants, is easy to recycle, and is an efficient catalyst.