The invention discloses a method for removing iron from a low-nickel sulfonium nitric acid leaching solution through pyrolysis. According to the method, the iron in the nickel/cobalt nitrate solution is removed through a high-temperature pyrolysis method; molten iron is decomposed into Fe2O3.H2O to be precipitated; the iron content of iron slag is greater than 50%, and through simple calcinations, the iron slag can be treated as iron ore concentrate; and the iron slag includes a small amount of valuable metal, and the recovery rate of nickel/cobalt is high. By means of the method, no impurity is introduced into the whole process; the procedure is simple; the application scope is wide; cost is low; subsequent purification can be conducted conveniently; generated nitric oxide can be recycled; and the method is a new environment-friendly iron-removal process which makes breakthrough.