The invention discloses a method for determining the content of single rear earth-included sulfur oxide in steel. The method comprises the following steps: soaking a steel sample with water and hydrochloric acid, and then cleaning and drying, wherein the mass of the steel sample is m1; taking the steel sample as an anode, taking stainless steel as a cathode, and taking an anhydrous methanol solution of anhydrous lithium chloride and acetylacetone as electrolyte for electrolysis; rinsing electrolytic residues with a mixed solution of iodine and absolute ethanol to obtain a first washing liquor, wherein the mass of the steel sample after electrolysis is m2; heating the electrolytic residues and the first washing liquor, and filtering to obtain a first precipitate; washing the first precipitate with water and nitric acid to collect the first precipitate and a first nitric acid washing liquor; heating the first precipitate and the first nitric acid washing liquor, and filtering to obtain a second precipitate; washing the second precipitate with nitric acid, combining the filtrate and the second nitric acid washing liquor to obtain a first test solution, and concentrating the first test solution and fixing the volume to obtain a test solution to be measured; determining masses m and m0 of single rear earth-included sulfur oxide in the test solution to be measured and a blank solution by an inductively coupled plasma mass spectrometer; calculating to obtain the content of single rear earth-included sulfur oxide.