The invention pertains to the field of water treatment technology and environmental functional materials, in particular to a catalyst used in composite three-dimensional electric field catalytic wet oxidation technique and a preparation method thereof. The catalyst takes Gamma-Al2O3 as a carrier, Mn metal oxide and Sn metal oxide as main active compounds and Sb oxide as an addition agent; wherein, the Mn metal oxide accounts for 65wt percent-67wt percent, the Sn metal oxide 32wt percent-34wt percent and the Sb oxide 0.5wt percent-1wt percent. Detail steps are that: the Gamma-Al2O3 carrier is soaked into the distilled water and then is dried for backup; the postprocessed Gamma-Al2O3 carrier is soaked into sol synthesized by Mn(NO3)2, SnCl4, SbCl3, HCl and C2H5, thus obtaining a Mn-Sn-Sb/Gamma-Al2O3 catalyst through dynamic soaking, baking, washing and drying. The invention has relatively good metal dispersivity of composite particles, even and subsequent crystal grains on the electrodesurface of particles, compact and tight crystal structure and relatively large roughness and valid ratio surface area which is larger than 199.6m<2>g; the catalyst is provided with relatively high catalytic activity.