A three-dimensional ordered macroporous lanthanum manganite supported platinum tin nanometer catalyst, and preparation method and application thereof belong to the technical field of automobile exhaust purification. The pore walls of a perovskite oxide LaMnO3 with a three-dimensional ordered macroporous structure is loaded with Pt-Sn nanoparticles in uniform size. The preparation method is as below: preparing a 3DOM LaMnO3 carrier by using a citrate assisted PMMA colloidal crystal template method, then loading platinum-tin alloy nanoparticles on the LaMnO3 surface by using an acid-modified PVA protection reduction method to obtain Pty-Sn / 3DOM LaMnO3. The catalyst has good three-way catalytic properties, and can effectively remove CO, HC and NOx in a reaction system. The present invention has the characteristics of cheap and readily available raw materials, simple preparation process, controllable morphology, particle size and pore size of the product and excellent catalytic performance.