A combined treatment method for laterite nickel ore hydrometallurgical slag and phosphating slag falls within the technical field of hydrometallurgy. The combined treatment method includes the steps uniformly mixing a laterite nickel ore hydrometallurgical slag, a phosphating slag, and a sodium alkaline salt to obtain a mixed material; subjecting the mixed material to sodium reduction roasting to obtain a roasted material; leaching the roasted material with water and filtering to obtain a water leaching solution and a water leaching slag; subjecting the water leaching slag to magnetic separation to obtain an iron concentrate. In the present invention, by mixing a laterite nickel ore hydrometallurgical slag and a phosphating slag and then performing sodium reduction roasting, it is ensured that the iron exists in the form of Fe3O4 and at the same time elements such as manganese and zinc exist in the slag in the form of oxides, and during the roasting process, aluminum oxides react with alkali to be converted into sodium aluminate which is readily soluble in water, and then high-efficiency separation of aluminum in the roasted material can be achieved by water leaching; at the same time, the reaction of zinc phosphide and manganese phosphide with alkali will promote the transformation of iron oxide from nonmagnetic to magnetic, and the subsequent magnetic separation process can achieve efficient recovery of iron.